Well since it's a plus problem I won't send you a debug version of Hopper to determine where the problem is...but I highly suspect the problem came from one line, which stores the running Messenger instance given by Plus!, of Hopper's Initialize subfunction...
I suggest you...
1. Try to close Windows Messenger's process (I see you are using version 6 which comes from Longhorn alpha and could be unstable) and see if the problem exists
2. Uninstall and reinstall MSN Messenger (unpatched)
3. Uninstall Plus! without preserving the settings and reinstall it. (But be aware, you may have to reinstall all of your plugins too)
Actually the best way is to call Patchou