Failed to connect to back-end dynamic database. Lync Server will continuously attempt to reconnect to the back-end. When ...

Failed to connect to back-end dynamic database.  Lync Server will continuously attempt to reconnect to the back-end. When user store mirror is configured, both the primary and mirror database servers will be tried until a successful connection is made to the active database server. While this condition persists, incoming messages will receive error responses. The most recent attempt failed with the below error.

Back-end Dynamic Database: %1  Connection string of: 
%2
Error: %3
Cause: Possible issues with back-end dynamic database.
Resolution:
Ensure the back-end is functioning correctly.