This issue occurs when Microsoft Lync 2013 is disconnected from a SIP gateway and then disconnected from a front-end server in a Lync Server 2010 or Lync Server 2013 environment. The disconnections may occur when Lync Server 2010 or Lync Server 2013 upgrades.
This issue occurs because lots of Lync 2013 clients try to sign in back and send subscriptions to the front-end server at the same time. Therefore, the front-end server has lots of load and takes a long time to deal with the sign-in requests.
To resolve this issue, install the following updates: