I have had this crash before on MindCrack v7. Solved it by disabling traincraft on the server. After that run the server, login, logout, then shutdown the server. Then re-enable traincraft. Be warned, this will remove all traincraft blocks and entities from the server. However it will fix the crash.
Then tell people not to use traincraft, or keep it disabled for now. Im waiting for v8 MindCrack to update traincraft, maybe it fixes the error. The error may also be caused by plugins such as BukkitForge. If your using that try updating the plugin.
