Can’t add new posts, categories, or comments…

I saved a new post and it disappeared into the void. Then I tried modifying an existing draft and it did save (and I eventually published it), but every time I try to add a category or tag it ignores them. When trying to publish a comment under the default commenting module, it returns a 404 page! Publishing comments using Disqus works, however.

This all happened about the same time my Wassup and Statpress plugins stopped returning data, detecting no hits to my site (from spam or anything). I tried removing all my plugins and reinstalling WordPress, and even corrected a couple errors in my database under the wp-comments table.

Any ideas?