Hi Ben,
Sorry for the delayed reply.
Microsoft states that this issue occurs because Lync for Mac 2011 calculates the expiration time of a web ticket based on an incorrect time zone. Therefore, when Lync for Mac 2011 tries to use the web ticket, the ticket is expired. For detailed information about it, please refer to http://support.microsoft.com/kb/2927775.
Microsoft has released the Microsoft Lync for Mac 2011 14.0.7 hotfix for solving this issue, but it may receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next update that contains this hotfix. To resolve this problem immediately, please refer to Description of the Microsoft Lync for Mac 2011 14.0.7 hotfix.
Regards,
Lee Li