Dunno.
Mac OS X is a closed book to me. I've had a quick go in PC world and gone "ooh" at the big icons in the dock, and that's about my lot.
Two things I'd do.
Firstly, see if there's some means of viewing the XML stream between your machine and the server. Many clients have a "debug" option of some sort.
Secondly, try another client and see if you get the same error.
[link|http://www.jabbercentral.org|http://www.jabbercentral.org] has a decent list of clients for all sorts of operating systems.