TLS negotiation failed with a incoming connection from an unknown Gateway. Remote Endpoint: %1 Reason: %2 Cause: A Gateway ...

TLS negotiation failed with a incoming connection from an unknown Gateway.

Remote Endpoint: %1
Reason: %2

Cause: A Gateway peer may not be configured to send connections to the port Mediation Server is listening to, or the certificate of the Gateway peer is not recognized by the Mediation Server.
Resolution:
Check that the Mediation server and Gateway certificates are configured correctly.  Check if the MEDIATIONSERVER_MAJOR_CONFIGURATION_ALARM (Event ID: 25057) has been fired.  Check whether the remote endpoint a known peer.