Internal Server Error, Help

again another crash ,i'm beggining to hate 1.4.2 ... . whenever someone tries to join my server it says 'Internal Server Error' this is what i get inside my console :

Please help me :z i can give all the information you need, when i restart my server it works again but afterwards when im somewhere else it happends again and there is none to restart it for me

anyone ?

EDIT by Moderator: merged posts, please use the edit button instead of double posting.