Client crashes immediately on server login

Since we have updated our clients and server to 1.5, whenever a player logs in to our server the client immediately crashes and produces the attached error report.
We noticed it only occurs if you log in in the spawn area - if you log in at an area outside of the spawn the client does not crash.
Wasn't sure if this was a Bukkit or Vanilla issue so posted it in both places.

If you want to see what happens, try logging in to magikpvp.com