The Group Chat Server is not trusted by the Lync Server. Reason string: %1 Error code: %2 Cause: This problem is usually ...

The Group Chat Server is not trusted by the Lync Server.

Reason string: %1

Error code: %2

Cause: This problem is usually caused by a wrong MTLS certificate configured on the Group Chat Server.
Resolution:
Review certificate related sections in Group Chat Planning and Deployment Guide. Ensure the MTLS certificate configured on the Group Chat Server is valid.