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

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

Cause: Either the A/V Authentication Service is not running or unreachable. The Lync Server may be down.
Resolution:
Verify that A/V Authentication Service is reachable from the Lync Server machine.