Some of the Trados GroupShare services fail to start, and the following Error message can be seen in the Trados GroupShare WebUI - System Configuration - Logs Viewer:

RabbitMQ.Client.Exceptions.BrokerUnreachableException: None of the specified endpoints were reachable ---> RabbitMQ.Client.Exceptions.AuthenticationFailureException: ACCESS_REFUSED - Login was refused using authentication mechanism PLAIN. For details see the broker logfile. at RabbitMQ.Client.Framing.Impl.Connection.StartAndTune() at ...

The Translation Model Service Host service configuration file needs to be edited and the RabbitMQ guest user password needs to be replaced with the correct one:

1. Go to C:\Program Files\SDL\SDL Server\TranslationModelService 
2. Search for the Sdl.TranslationModelService.Host.exe.config file and open it with a file editor like Notepad++
3. On the opened file, search for the following command line : <credential key="RabbitMQ" userName="guest" password="PasswordHere=" />
4. Replace the PasswordHere field with the correct RabbitMQ guest password from a working Trados GroupShare service .config file



5. The Trados GroupShare services and their .config files can be found at this location: C:\Program Files\SDL\SDL Server (this is the Trados GroupShare's default services and .config files location)
6. Restart the affected Trados GroupShare service

NOTE! If you are looking for the BCM Service config file, on a newer than CU6 version of Trados GroupShare 2020 SR1, you should find it at this path on the application server:
C:\Program Files\SDL\SDL Server\BCM Service\appsettings.json. The previous path was this one: C:\Program Files\SDL\SDL Server\BCM Service\Sdl.BcmService.Host.exe.config.

OBS! The Translation Model Service Host service is an example of a Trados GroupShare service. Of course, this fix can be applied to other services too.
OBS2! In some situations this resolution solves the service not starting issue, and there might appear some errors in the Trados GroupShare WebUI - System Configuration - Logs Viewer. 
OBS3! There are situations where this workaround is not fixing the issue.

The RabbitMQ login credentials for the guest user were changed and the Translation Model Service Host service does not start anymore.