When I join my server, I've got this error :
http://pastebin.com/J94KL9sb

Thanks for your help.

It looks like someone attached a balloon to an entity (?Mod entity) and that ?mod doesn't like it.
You can backup your world and try and remove the chunk that has this balloon with MCEdit and see it that solves your problem.

What version is this?

@Tickbuz is actually right...someone latched it onto a mod entity, and the mod entity is still referencing all living entities as EntityLiving instead of the new EntityLivingBase.