Consider the following scenario:
A proxy server that requires all outgoing web access requests to be authenticated is deployed in an organization.
You sign in to Microsoft Lync 2010.
A Lync 2010 user in a non-federated organization sends you a meeting invitation. The invitation enables anonymous users to join the meeting.
You try to join the meeting.
In this scenario, you encounter a 407 error from the proxy server. Additionally, you receive the following error message:
Cannot sign in because the server is temporarily unavailable.
error ID 28 (source ID 238).
To resolve this issue, install the following cumulative update:
2842627 Description of the cumulative update package for Lync 2010: July 2013
Note After you install the update, if you try to join a meeting, you are prompted to enter credentials.