Consider the following Scenario:
You log on to Microsoft Lync Online.
You save the logon password.
You log off Lync Online.
You delete the credentials that are used to connect to Lync Online in Windows Credential Manager.
You log on to Lync Online again.
In this scenario, Lync 2010 does not prompt you for credentials.
Note This issue specifically occurs when you have a Session Initiation Protocol (SIP) certificate that is saved locally.
To resolve this issue, install the following cumulative update:
2496325 Description of the cumulative update package for Lync 2010: April 2011
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.