I've had this problem before, and the only solution I found is to reinstall WordPress completely. The problem is - after clicking on plugins tab the page never opens "loading..." infinitely (no error message either). I did change permissions to everything inside wp-content and the folder itself to 755 or even 777. Nothing.
The only plugin installed is Sidebar Widgets, and it does work. So now I can neither add new plugins, nor disable existing. Please, help! The site is online and reinstalling again is not a rosy option.
I do suspect it might be something in server/host configuration, because I do not have this problem in my test setup in local PC.