A 21Vianet user may be repeatedly prompted for Exchange credentials after signing into the Lync client

Symptoms

Consider the following scenario:

• You are a customer of Microsoft Office 365 operated by 21Vianet.

• You successfully sign into the Lync service.

In this scenario, some users are repeatedly prompted to enter Exchange credentials. 

Cause

This issue occurs because the Lync client does not properly obtain the federated partner in certain authentication paths in Office 365 operated by 21Vianet.  
 

Resolution

This issue has been fixed in the April 2014 update of the Lync client.