Hi Herbert,
I am also getting the same alert in our SfB Environment. In my SfB environment Front End and the Mediation servers are co-located.
Like you mentioned in your earlier reply enabling and tracing the logs is not possible because it occues randomly in a day. The users are not having any complaints about Call failures. So i am not sure why the following alert occurs.
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Date and Time: 10/27/2016 5:43:35 PMLog Name: Lync Server
Source: LS Mediation Server
Event Number: 25067
Level: 1
Logging Computer: SERVER01
User: N/A
Description:
The Mediation Server service has encountered a major call completion problem with the Front End. Front End FQDN: SERVER01
Cause: Calls to this Front End failed 5 times with failure final responses. Check other MOM alerts for more details.
Resolution: If the failure is due to CALL_FAILURES_DUE_TO_UNEXPECTED_PROXY_INTERACTION, there is some internal issue with the Front End's link to the Mediation Server (misconfiguration, load) that is causing it not to complete calls. Please consult the failure reports on the Monitoring Server for more details.
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Regards,
Yogesh S.