Image uploader works for one blog and not the other

Just updated two WP blogs running on two different servers, to 2.8.4. On one blog the uploader works, on the other it doesn’t. It doesn’t matter what browser I use, or even what operating system.

I deactivated all control panel plugins or anything that could be related, and it had no effect. (I’m not running a lot of plugins: akismet, backup, Sobek’s posts to show list of posts in a category, PodPress 8.8, smartYouTube, tag cloud.)

I know it isn’t a flash issue in the browser because it works fine for one blog and not the other. The blog for which it does NOT work has all flash failing in the cp, including screen settings so I can’t even change those. Interestingly, the flash works in OTHER cp pages. Just not in edit.php or post-new.php. There is no difference between the files in the two installations. What else should I look at?

I did a cursory comparison of the two installations, especially swf-uploader and js, and see nothing amiss between them, both installations seem to have correctly updated (used the WP built-in automated updater.)

Any ideas?

Should I do a complete, manual reinstall of the wp-includes and wp-admin folders?