One possible solution which is to refresh Lync cached endpoints. 

When Lync 2010 connects to a specific front-end server, it caches that endpoint to make the sign-in process faster in the future. However, sometimes the endpoint can be changed which can cause your user sign-in to fail. To delete the endpoint cache, follow these steps:

Locate the local application data folder:

Windows Vista and Windows 7: %LOCALAPPDATA%\Microsoft\Communicator\<sip_address@contoso.com>\

Windows XP: %USERPROFILE%\Local Settings\Application Data\Microsoft\Communicator\<sip_address@contoso.com>\

Delete the file that is named EndpointConfiguration.CACHE.

Restart Lync 2010, and then try to sign in to Lync Online.