When you sign in to a Microsoft Lync 2010 client by using the Lync Online service, credential prompts are displayed again. This issue occurs even after you type valid credentials.
To resolve this issue, install the following cumulative update:
2540951 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.