Weird problem affecting users differently
 

Starting today, some users have reported a problem posting text over several lines. Some of the same users have trouble attaching images.

Other users are fine.

I asked them which browsers they used, and both for IE, Firefox and Safari some were having problems, others not.

I reverted a few templates that had been postponed from my latest upgrade two weeks ago, and it made no difference to any of the users having problems.

I discovered that I myself could not expand the menu on the administrator's control panel. It just won't expand.

I tried the same at another forum which I have administrator access to, and there were no problems there, even when that was in another tab in the same browserwindow where I couldn't access my own forum. I tried my own forum in another browser (and with another username), and the menu on the administrator's control panel still won't expand.

How should I troubleshoot this?