That sounds a lot like a compliance server enforcing policy. Do you have one integrated with IM&P (e.g. Acitance, Verba, etc.)?
If not, can you log that user back into an earlier Jabber release and get it working again? What about logging an otherwise working user into the workstation this user is having the problem with? In other words: does this problem follow the computer/Jabber install, the user when using Jabber 12.1, or the user across all versions of Jabber?