widgets not working

I’ve seen this problem addressed in other threads, always with the same advice: turn off plug-ins. Unfortunately, that does not work for me; even with all plug-ins turned off, widgets still do not work properly in the Administrative section.

I am fairly sure everything was fine until I added a Video Widget and a Gallery Stream Widget to the Sidebar Page. I had previously added these two widgets to the Sidebar Single, with no problem. Since adding them to the Sidebar Page, I cannot do anything on the Widget page, like opening and closing widgets or even accessing Screen Options.