In “QPM Admin | General Settings | Realm Instances “ the following error may be displayed
"System error occurred. For more information, view the event log.
Failed to remove the service instance.
Task with id='ac5cda12-3376-49f4-842d-14d925192fcf' is not registered"
A wrong certificate is written in Local.Storage file.
The old certificate is no longer on the machine, so QPM Service cannot correctly start its WCF listeners.
The certificate listed in Local.Storage file is not valid (it would be selected on UI otherwise).
Delete the certificate ID from thumbprint in Local.Storage file. Search for "<setting name="certificateThumbprint" value="<XXXX>" />"
Restart QPM Service.