Consider the following scenario:
User A and user B joins an Instant Message (IM) conference by using Microsoft Lync 2010.
An SQL Back-end failover occurs.
During and after the failover, the conference roster is changed because of an action. For example, a user joins or leaves the conference.
In this scenario, the conference roster of user A and user B is not updated, and they do not know whether a user has joined or left.
This issue occurs because the Lync 2010 client receives a 488 not acceptable here response, and then the client does not rejoin the focus conferencing component.
To resolve this issue, install the following cumulative update:
2514982 Description of the cumulative update for Lync 2010: November 2011
Note after the update is applied, Lync 2010 rejoins the focus conferencing component when the client receives the 488 not acceptable here response.