This issue occurs when a user joins a meeting anonymously by using a Microsoft Lync client in a Lync Server 2013 front-end pool (pool A). The meeting is hosted in a Lync Server 2013 front-end pool (pool B) that has multiple front-end servers deployed. Pool A and pool B are not federated.
When this issue occurs, anonymous users cannot participate in a chat, cannot use other modality in the meeting, or may be kicked out of the meeting.
This issue occurs because the Edge server of pool B sends a request to a random front-end server that is not the original authentication server in pool A.
To resolve this issue, install the February 2015 Cumulative Update for Lync Server 2013 (Front End Server and Edge Server).