The first thing to do here is to look in the application event log for error messages - when you get that failsafe message (i.e. the "I'm can't talk to you now..." prompt) - there will always be one or more errors written to the application event log. This will give us a clue as to where to start.
Most likely it's a problem logging into that users mailbox - checking to be sure the message store facing account for the Unity services has rights to SA/RA for the mailstore that user is in and the like are areas to look at next - but seeing what error we're getting back would be the first place you want to start.
