Ajax on Sub directory don't work


I have just maked a new subdomain, and it strongly with that ajax in the admin part does not work on the sub sites. It works find on the head domain (domain.com/wp-admin/)

But when I by example want to add a category to the the menu, in appearance-> menu, the ring just keep on going.

I then tried too see if I could see what was wrong in the firebug, and I can see it comes up with a "500 Internal Server Error"

Any ideas ?