The A/V Authentication Service cannot be contacted. Connections that require Firewall traversal will not be successful. Mediation ...

The A/V Authentication Service cannot be contacted. Connections that require Firewall traversal will not be successful. Mediation server will keep trying; however additional failures will not be logged.

Cause: Either the A/V Authentication Service is not running or unreachable.
Resolution:
Verify that A/V Authentication Service is reachable from the computer running Lync Server.