In fact, as reported CUCM version 6 is very old and not supported for almost 10 years which limits the search scope of the solution.
What I would point out to you is:
Get a screenshot of the error;
Collect CUCM logs via RTMT tool to try to understand in detail why the service is down;
Evaluate if something has been modified in CUCM to change its MAC Address (Component that links the license to Publisher). Download the license, open with Notepad and verify that the HOSTID is the same as the MAC Address of the CUCM.
