Can't join a meeting when proxy authentication is required in Lync 2010

Symptoms

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:

Resolution

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.