http://pastebin.com/n4BLtNts
Version: 0.7.0a

Could you tell me what is causing it ?

Update to v0.7.2a to fix this issue. It was due to the Elementals attempting to divide by 0. :D

updating didn´t fix the world :(

I think you will need to use some word-editing software to delete all the entities in the world.

Is there a brief few seconds where everything is working before the freeze?

Wait... Had a better idea. Forge happens to have something for just such an occasion! Go to your configs and find forge, open it up and change "B:removeErroringEntities=false" to true. This might fix it, or break everything and cause a rip in space-time. Not really sure at this point...

yeah setting the world to peaceful didn´t work either.

kay. the server freezes instant, I did a small rollback.