Have you tried:
– deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).
– switching to the Twenty Eleven theme to rule out any theme-specific problems.
– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
– re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress.
Thanks esmi!
It appears the problem was with my theme, which was no longer compatible with the WP core after one of the WP upgrades. It was so subtle that I didn’t notice it right away since everything appeared to work, and everything looked correct. (I didn’t even know my theme could have an affect on wp-admin pages!)
I think it was a conflict with some of the scripts, or a javascript library or something. But I upgraded my theme and now everything works like peaches! 🙂