On Foreman 1.15.2/Katello 3.4.4, I'm creating a host behind a proxy content and it fails to exit build mode:

$ curl http://myproxy.xxx:8000/unattended/built?token=75b56b02-687c-47e0-9bc8-9e80bfc62e1a
Failed to retrieve built template for {"token"=>"75b56b02-687c-47e0-9bc8-9e80bfc62e1a", "splat"=>[], "captures"=>["built"], "kind"=>"built"}: Error retrieving built for {"token"=>"75b56b02-687c-47e0-9bc8-9e80bfc62e1a", :url=>"http://myproxy.xxx:8000"} from mymaster.xxx: Net::HTTPConflict

In Foreman logs:

2017-08-04 12:17:44 7faec60a [app] [I] Started GET "/unattended/built?token=75b56b02-687c-47e0-9bc8-9e80bfc62e1a&url=http%3A%2F%2Fmyproxy.xxx%3A8000" for myproxy.xxx at 2017-08-04 12:17:44 +0000
2017-08-04 12:17:44 7faec60a [app] [I] Processing by UnattendedController#built as TEXT
2017-08-04 12:17:44 7faec60a [app] [I]   Parameters: {"token"=>"75b56b02-687c-47e0-9bc8-9e80bfc62e1a", "url"=>"http://myproxy.xxx:8000", "unattended"=>{}}
2017-08-04 12:17:44 7faec60a [app] [I] Current user: foreman_api_admin (administrator)
2017-08-04 12:17:44 7faec60a [app] [I] unattended: dena-ikner.cfc.ks.net is Built!
2017-08-04 12:17:46 7faec60a [app] [I] Delete the DNS A record for dena-ikner.cfc.ks.net/172.29.14.32
2017-08-04 12:17:47 7faec60a [app] [I] Delete the DNS PTR record for 172.29.14.32/dena-ikner.cfc.ks.net
2017-08-04 12:17:47 7faec60a [app] [I] Add DNS A record for dena-ikner.cfc.ks.net/172.29.14.16
2017-08-04 12:17:47 7faec60a [app] [I] Add DNS PTR record for 172.29.14.16/dena-ikner.cfc.ks.net
2017-08-04 12:17:47 7faec60a [app] [W] Create Reverse IPv4 DNS record for dena-ikner.cfc.ks.net task failed with the following error: ERF12-2357 [ProxyAPI::ProxyException]: Unable to set DNS entry ([RestClient::Conflict]: 409 Conflict) for proxy http://mydnsproxy.xxx:8000/dns
 | ProxyAPI::ProxyException: ERF12-2357 [ProxyAPI::ProxyException]: Unable to set DNS entry ([RestClient::Conflict]: 409 Conflict) for proxy http://mydnsproxy.xxx:8000/dns
 | /usr/share/foreman/lib/proxy_api/dns.rb:15:in `rescue in set'
 | /usr/share/foreman/lib/proxy_api/dns.rb:13:in `set'
 | /usr/share/foreman/lib/net/dns/reverse_record.rb:20:in `create'
 | /usr/share/foreman/app/models/concerns/dns_interface.rb:43:in `set_dns_record'
 | /usr/share/foreman/app/models/concerns/orchestration.rb:215:in `execute'
 | /usr/share/foreman/app/models/concerns/orchestration.rb:144:in `block in process'
 | /usr/share/foreman/app/models/concerns/orchestration.rb:136:in `each'
 | /usr/share/foreman/app/models/concerns/orchestration.rb:136:in `process'
 | /usr/share/foreman/app/models/concerns/orchestration.rb:44:in `around_save_orchestration'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:432:in `block in make_lambda'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:312:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:312:in `block in halting'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:497:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:497:in `block in around'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:92:in `__run_callbacks__'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:778:in `_run_save_callbacks'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/callbacks.rb:302:in `create_or_update'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/persistence.rb:120:in `save'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/validations.rb:37:in `save'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/attribute_methods/dirty.rb:21:in `save'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/transactions.rb:286:in `block (2 levels) in save'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/transactions.rb:351:in `block in with_transaction_returning_status'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/abstract/database_statements.rb:211:in `transaction'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/transactions.rb:220:in `transaction'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/transactions.rb:348:in `with_transaction_returning_status'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/transactions.rb:286:in `block in save'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/transactions.rb:301:in `rollback_active_record_state!'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/transactions.rb:285:in `save'
 | /usr/share/foreman/app/models/concerns/foreman/sti.rb:29:in `save_with_type'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/foreman-tasks-0.9.4/app/models/foreman_tasks/concerns/action_triggering.rb:25:in `block in save_with_dynflow_task_wrap'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/foreman-tasks-0.9.4/app/models/foreman_tasks/concerns/action_triggering.rb:106:in `dynflow_task_wrap'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/foreman-tasks-0.9.4/app/models/foreman_tasks/concerns/action_triggering.rb:25:in `save_with_dynflow_task_wrap'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/persistence.rb:252:in `block in update'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/transactions.rb:351:in `block in with_transaction_returning_status'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/abstract/database_statements.rb:213:in `block in transaction'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/abstract/transaction.rb:184:in `within_new_transaction'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/abstract/database_statements.rb:213:in `transaction'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/transactions.rb:220:in `transaction'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/transactions.rb:348:in `with_transaction_returning_status'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/persistence.rb:250:in `update'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/foreman-tasks-0.9.4/app/models/foreman_tasks/concerns/action_triggering.rb:37:in `block in update_attributes'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/foreman-tasks-0.9.4/app/models/foreman_tasks/concerns/action_triggering.rb:119:in `dynflow_task_wrap'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/foreman-tasks-0.9.4/app/models/foreman_tasks/concerns/action_triggering.rb:37:in `update_attributes'
 | /usr/share/foreman/app/controllers/unattended_controller.rb:192:in `update_ip'
 | /usr/share/foreman/app/controllers/unattended_controller.rb:28:in `built'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_controller/metal/implicit_render.rb:4:in `send_action'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/abstract_controller/base.rb:198:in `process_action'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_controller/metal/rendering.rb:10:in `process_action'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/abstract_controller/callbacks.rb:20:in `block in process_action'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:117:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:117:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:555:in `block (2 levels) in compile'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:498:in `block (2 levels) in around'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:313:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:313:in `block (2 levels) in halting'
 | /usr/share/foreman/app/controllers/concerns/application_shared.rb:15:in `set_timezone'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:432:in `block in make_lambda'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:312:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:312:in `block in halting'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:497:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:497:in `block in around'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:498:in `block (2 levels) in around'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:313:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:313:in `block (2 levels) in halting'
 | /usr/share/foreman/app/models/concerns/foreman/thread_session.rb:32:in `clear_thread'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:432:in `block in make_lambda'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:312:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:312:in `block in halting'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:497:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:497:in `block in around'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:498:in `block (2 levels) in around'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:313:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:313:in `block (2 levels) in halting'
 | /usr/share/foreman/app/controllers/concerns/foreman/controller/topbar_sweeper.rb:12:in `set_topbar_sweeper_controller'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:432:in `block in make_lambda'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:312:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:312:in `block in halting'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:497:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:497:in `block in around'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:498:in `block (2 levels) in around'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:313:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:313:in `block (2 levels) in halting'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/audited-4.4.1/lib/audited/sweeper.rb:14:in `around'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:455:in `public_send'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:455:in `block in make_lambda'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:312:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:312:in `block in halting'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:497:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:497:in `block in around'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:505:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:92:in `__run_callbacks__'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:778:in `_run_process_action_callbacks'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:81:in `run_callbacks'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/abstract_controller/callbacks.rb:19:in `process_action'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_controller/metal/rescue.rb:29:in `process_action'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_controller/metal/instrumentation.rb:32:in `block in process_action'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/notifications.rb:164:in `block in instrument'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/notifications.rb:164:in `instrument'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_controller/metal/instrumentation.rb:30:in `process_action'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_controller/metal/params_wrapper.rb:250:in `process_action'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/railties/controller_runtime.rb:18:in `process_action'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/abstract_controller/base.rb:137:in `process'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionview-4.2.5.1/lib/action_view/rendering.rb:30:in `process'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_controller/metal.rb:196:in `dispatch'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_controller/metal/rack_delegation.rb:13:in `dispatch'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_controller/metal.rb:237:in `block in action'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/routing/route_set.rb:74:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/routing/route_set.rb:74:in `dispatch'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/routing/route_set.rb:43:in `serve'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/journey/router.rb:43:in `block in serve'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/journey/router.rb:30:in `each'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/journey/router.rb:30:in `serve'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/routing/route_set.rb:815:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/static.rb:116:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/static.rb:116:in `call'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/apipie-rails-0.4.0/lib/apipie/static_dispatcher.rb:65:in `call'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/apipie-rails-0.4.0/lib/apipie/extractor/recorder.rb:132:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/static.rb:116:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/static.rb:116:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/static.rb:116:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/static.rb:116:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/static.rb:116:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/static.rb:116:in `call'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/apipie-rails-0.4.0/lib/apipie/middleware/checksum_in_headers.rb:27:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/etag.rb:24:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/conditionalget.rb:25:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/head.rb:13:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/params_parser.rb:27:in `call'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/katello-3.4.4/lib/katello/params_parser_wrapper.rb:12:in `call'
 | /usr/share/foreman/lib/middleware/catch_json_parse_errors.rb:8:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/flash.rb:260:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/session/abstract/id.rb:225:in `context'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/session/abstract/id.rb:220:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/cookies.rb:560:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/query_cache.rb:36:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:653:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/callbacks.rb:29:in `block in call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:88:in `__run_callbacks__'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:778:in `_run_call_callbacks'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/callbacks.rb:81:in `run_callbacks'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/callbacks.rb:27:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/remote_ip.rb:78:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/debug_exceptions.rb:17:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/show_exceptions.rb:30:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/railties-4.2.5.1/lib/rails/rack/logger.rb:38:in `call_app'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/railties-4.2.5.1/lib/rails/rack/logger.rb:22:in `call'
 | /usr/share/foreman/lib/middleware/tagged_logging.rb:18:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/request_id.rb:21:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/methodoverride.rb:22:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/runtime.rb:18:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/activesupport-4.2.5.1/lib/active_support/cache/strategy/local_cache_middleware.rb:28:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/actionpack-4.2.5.1/lib/action_dispatch/middleware/static.rb:116:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/sendfile.rb:113:in `call'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/secure_headers-3.4.1/lib/secure_headers/middleware.rb:12:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/railties-4.2.5.1/lib/rails/engine.rb:518:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/railties-4.2.5.1/lib/rails/application.rb:165:in `call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/railties-4.2.5.1/lib/rails/railtie.rb:194:in `public_send'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/railties-4.2.5.1/lib/rails/railtie.rb:194:in `method_missing'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/urlmap.rb:66:in `block in call'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/urlmap.rb:50:in `each'
 | /opt/rh/sclo-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/urlmap.rb:50:in `call'
 | /usr/share/passenger/phusion_passenger/rack/thread_handler_extension.rb:74:in `process_request'
 | /usr/share/passenger/phusion_passenger/request_handler/thread_handler.rb:141:in `accept_and_process_next_request'
 | /usr/share/passenger/phusion_passenger/request_handler/thread_handler.rb:109:in `main_loop'
 | /usr/share/passenger/phusion_passenger/request_handler.rb:455:in `block (3 levels) in start_threads'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/logging-1.8.2/lib/logging/diagnostic_context.rb:323:in `call'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/logging-1.8.2/lib/logging/diagnostic_context.rb:323:in `block in create_with_logging_context'
2017-08-04 12:17:47 7faec60a [app] [I] Delete the DNS A record for dena-ikner.cfc.ks.net/172.29.14.16
2017-08-04 12:17:47 7faec60a [app] [I] Add DNS PTR record for 172.29.14.32/dena-ikner.cfc.ks.net
2017-08-04 12:17:48 7faec60a [app] [I] Add DNS A record for dena-ikner.cfc.ks.net/172.29.14.32
2017-08-04 12:17:49 7faec60a [app] [I] Completed 409 Conflict in 4231ms (ActiveRecord: 31.9ms)

It seems that it tries to re-create the DNS record (already created at the beginning of the build).

If I run the request on the master it works as expected:

$ curl http://mymaster.xxx/unattended/built?token=75b56b02-687c-47e0-9bc8-9e80bfc62e1a
$
2017-08-04 12:18:51 29ada839 [app] [I] Started GET "/unattended/built?token=75b56b02-687c-47e0-9bc8-9e80bfc62e1a" for 10.2.3.4 at 2017-08-04 12:18:51 +0000
2017-08-04 12:18:51 29ada839 [app] [I] Processing by UnattendedController#built as TEXT
2017-08-04 12:18:51 29ada839 [app] [I]   Parameters: {"token"=>"75b56b02-687c-47e0-9bc8-9e80bfc62e1a"}
2017-08-04 12:18:51 29ada839 [app] [I] Current user: foreman_api_admin (administrator)
2017-08-04 12:18:51 29ada839 [app] [I] unattended: dena-ikner.cfc.ks.net is Built!
2017-08-04 12:18:54 29ada839 [app] [I] Completed 201 Created in 2987ms (ActiveRecord: 143.0ms)