menu edit problem

hello,

In menu edit I can’t unfold or fold categories, pages etc. I clicking on it but nothing happens, pages are unfold, so I could add them, but I can’t get to categories. I have lots of categories and previously I had added them without problem.
Screenshot with marked problem: https://www.dropbox.com/s/xbhpneajbz83oc8/wordpress.jpg

I have wordpress 3.6–pl_PL, and tested it in chrome and firefox with the same problem.