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\<email@example.com>\
Windows XP: %USERPROFILE%\Local Settings\Application Data\Microsoft\Communicator\<firstname.lastname@example.org>\
Delete the file that is named EndpointConfiguration.CACHE.
Restart Lync 2010, and then try to sign in to Lync Online.