Consider the following scenario:
You sign in to Microsoft Lync 2010 in an organization that has a proxy server deployed.
You try to join a meeting that is created by a user in a non-federated organization.
A proxy authentication dialog box appears.
You input proxy authentication credentials.
In this scenario, the proxy authentication fails and you cannot join the meeting.
This issue occurs because a redundant backslash (\) is added before the user name when Lync 2010 connects to the proxy server.
Note The redundant backslash can only be observed in Network Trace.
To resolve this issue, install the following cumulative update:
2842627 Description of the cumulative update package for Lync 2010: July 2013