Hello, I am still encountering this issue:

The action raised an exception [action_ex_id=bc158ba8-ea5e-42d0-9712-30d24698a364, action_cls='<class 'mistral.actions.action_factory.GetOvercloudConfig'>', attributes='{}', params='{u'container_config': u'overcloud-config', u'container': u'overcloud'}']
 u'23f1abe0-aa3f-46c3-a712-d3216b9b297d'


[stack@undercloud (stackrc) ~]$ mistral execution-get dc662a9e-91e6-45a2-a033-0300e7bdc386                            
+--------------------+---------------------------------------+
| Field              | Value                                 |
+--------------------+---------------------------------------+
| ID                 | dc662a9e-91e6-45a2-a033-0300e7bdc386  |
| Workflow ID        | f3401518-86d3-4c8a-aad7-0507362bcf97  |
| Workflow name      | tripleo.messaging.v1.send             |
| Workflow namespace |                                       |
| Description        | sub-workflow execution                |
| Task Execution ID  | 9e060c2a-f011-40ff-bab2-18de8a85fa1d  |
| Root Execution ID  | bd5a619c-4ba4-495e-938d-f238f027e250  |
| State              | ERROR                                 |
| State info         | Workflow failed due to message status |
| Created at         | 2018-11-21 18:47:35                   |
| Updated at         | 2018-11-21 18:47:39                   |
+--------------------+---------------------------------------+
[stack@undercloud (stackrc) ~]$ mistral task-get 9e060c2a-f011-40ff-bab2-18de8a85fa1d
+-----------------------+----------------------------------------------+
| Field                 | Value                                        |
+-----------------------+----------------------------------------------+
| ID                    | 9e060c2a-f011-40ff-bab2-18de8a85fa1d         |
| Name                  | send_message                                 |
| Workflow name         | tripleo.deployment.v1.config_download_deploy |
| Workflow namespace    |                                              |
| Workflow Execution ID | bd5a619c-4ba4-495e-938d-f238f027e250         |
| State                 | ERROR                                        |
| State info            | Workflow failed due to message status        |
| Created at            | 2018-11-21 18:47:35                          |
| Updated at            | 2018-11-21 18:47:39                          |
+-----------------------+----------------------------------------------+
[stack@undercloud (stackrc) ~]$ mistral execution-get  bd5a619c-4ba4-495e-938d-f238f027e250
+--------------------+-----------------------------------------------------------------------------------------------------------+
| Field              | Value                                                                                                     |
+--------------------+-----------------------------------------------------------------------------------------------------------+
| ID                 | bd5a619c-4ba4-495e-938d-f238f027e250                                                                      |
| Workflow ID        | 29d5cd71-4b32-4dfc-9710-38a213b2e8e6                                                                      |
| Workflow name      | tripleo.deployment.v1.config_download_deploy                                                              |
| Workflow namespace |                                                                                                           |
| Description        |                                                                                                           |
| Task Execution ID  | <none>                                                                                                    |
| Root Execution ID  | <none>                                                                                                    |
| State              | ERROR                                                                                                     |
| State info         | Failure caused by error in tasks: send_message                                                            |
|                    |                                                                                                           |
|                    |   send_message [task_ex_id=9e060c2a-f011-40ff-bab2-18de8a85fa1d] -> Workflow failed due to message status |
|                    |     [wf_ex_id=dc662a9e-91e6-45a2-a033-0300e7bdc386, idx=0]: Workflow failed due to message status         |
|                    |                                                                                                           |
| Created at         | 2018-11-21 18:46:12                                                                                       |
| Updated at         | 2018-11-21 18:47:41                                                                                       |
+--------------------+-----------------------------------------------------------------------------------------------------------+


Logs observed on undercloud:

/var/log/container/engine.log: 
./engine.log:2018-11-21 18:47:39.968 7 INFO workflow_trace [req-38c24156-db63-4fae-9f2d-cf389ad8b04c fc9c6785739f40c7bfa7c22621f4ddbf 3a971c7740294e00b8109a7d12b7f471 - default default] Task 'send_message' (9e060c2a-f011-40ff-bab2-18de8a85fa1d) [RUNNING -> ERROR, msg=Workflow failed due to message status] (execution_id=bd5a619c-4ba4-495e-938d-f238f027e250)
./engine.log:2018-11-21 18:47:41.068 7 INFO workflow_trace [req-525265a7-4cbd-48dc-8de6-6c707e2f25a9 - - - - -] Workflow 'tripleo.deployment.v1.config_download_deploy' [RUNNING -> ERROR, msg=Failure caused by error in tasks: send_message


Most relevant log error Ive found so far: (the UUID is not the UUID of the scaled-out compute – but this error occurs during scale-out at the same timestamp above) 

/var/log/container/executor.log:

2018-11-21 18:47:35.380 7 WARNING mistral.executors.default_executor [req-eb5348cd-bd3c-4ea6-9aac-5653a0942c0d fc9c6785739f40c7bfa7c22621f4ddbf 3a971c7740294e00b8109a7d12b7f471 - default default] The action raised an exception [action_ex_id=bc158ba8-ea5e-42d0-9712-30d24698a364, action_cls='<class 'mistral.actions.action_factory.GetOvercloudConfig'>', attributes='{}', params='{u'container_config': u'overcloud-config', u'container': u'overcloud'}']
 u'23f1abe0-aa3f-46c3-a712-d3216b9b297d': KeyError: u'23f1abe0-aa3f-46c3-a712-d3216b9b297d'
2018-11-21 18:47:35.380 7 ERROR mistral.executors.default_executor Traceback (most recent call last):
2018-11-21 18:47:35.380 7 ERROR mistral.executors.default_executor   File "/usr/lib/python2.7/site-packages/mistral/executors/default_executor.py", line 114, in run_action
2018-11-21 18:47:35.380 7 ERROR mistral.executors.default_executor     result = action.run(action_ctx)
2018-11-21 18:47:35.380 7 ERROR mistral.executors.default_executor   File "/usr/lib/python2.7/site-packages/tripleo_common/actions/config.py", line 76, in run
2018-11-21 18:47:35.380 7 ERROR mistral.executors.default_executor     commit_message=message)
2018-11-21 18:47:35.380 7 ERROR mistral.executors.default_executor   File "/usr/lib/python2.7/site-packages/tripleo_common/utils/config.py", line 424, in download_config
2018-11-21 18:47:35.380 7 ERROR mistral.executors.default_executor     self.write_config(stack, name, config_dir, config_type)
2018-11-21 18:47:35.380 7 ERROR mistral.executors.default_executor   File "/usr/lib/python2.7/site-packages/tripleo_common/utils/config.py", line 298, in write_config
2018-11-21 18:47:35.380 7 ERROR mistral.executors.default_executor     server_names[server_id],
2018-11-21 18:47:35.380 7 ERROR mistral.executors.default_executor KeyError: u'23f1abe0-aa3f-46c3-a712-d3216b9b297d'






This code DOES exist in in our codebase – however this looks like the same exact issue. 

Looking at the blacklisted servers during scale out :

[stack@undercloud (stackrc) mistral]$ cat /home/stack/templates/server-blacklist.yaml
parameter_defaults:
  DeploymentServerBlacklist:
    - overcloud-ovscompute-1
    - overcloud-ovscompute-0

Comparing the Keyerror uuid above to nova list:
	Belongs to overcloud-ovscompute-0
23f1abe0-aa3f-46c3-a712-d3216b9b297d | overcloud-ovscompute-0