Try deactivating all plugins and check to see if this resolves the problem. If it does, re-activate the plugins one by one until you find the problematic plugin(s).
Also see:
http://wordpress.org/support/topic/279132
Have you tried enabling accessability mode?
In the widgets admin area, upper-right, click Screen Options, then Upper-Left, click enable accessability mode.
Removing the plugins didn’t work
with accessability mode i was able to add widgets without removing the others but they still don’t appear in the widget list so i cant edit/remove them.
I found the solution (a little while ago already)
I messed with some code, after downloading the theme again and removing the old one it worked again.