Cannot sign in to Communicator. You may have entered your sign-in address, user name, or password incorrectly, or the authentication service may be incompatible with this version of the program. If your sign-in information is correct and the problem persists, please contact your system administrator. 

One quick thing you can check is on the Communications tab on the user properties. Under "additional options: Configure", check that "Enable remote user access" is ticked. If this is not ticked, the user will receive the error above (instead of a USEFUL message like "This account is not enabled for remote access .... ").