When this issue occurs on a Microsoft Lync Server 2013 front-end server, client users may encounter the following issues:
New users cannot sign in to the front-end server.
Users cannot complete instant message conversations.
Users cannot join meetings that are hosted on the front-end server.
Additionally, when you restart the Front End service (RTCSrv.exe), the service freezes.
To resolve this issue, install the July 2015 cumulative update 5.0.8308.920 for Lync Server 2013 (Front End Server and Edge Server).
This issue occurs because of a race condition in the Front End service.