I also tried removing my entire wordpress installation only blacking up posts and uploads and again newly made my wordpress blog but the same problem! is it a problem from my host! I also tried making chmod 755 to widgets.php but not solved!
I had a plug-in cause that exact kind of problem before. Can’t remember the name of the plugin, but it was one that pulled in feeds from Yahoo I believe….Anyway, try to disable plug-ins one at a time & try it after each removal.
I had a similar problem… I installed the Google Libraries Plugin and it fixed it:
I guess it was some sort of Javascript bug stopping JS from loading properly on the widgets page…
Good luck!