blank wp-comments-post.php error

Sorry, I know there are other posts by this title, but either the topics are already closed, or it doesn’t really help with my problem.

First of all, I’m using WordPress 2.1 and testing in both Firefox and IE. I’m not using any plugins at all and I’ve already tried switching themes.

After upgrading to 2.1 (sorry, I don’t remember what the previous version was), no one can make any comments on my posts, they just get taken to a blank page (wp-comments-post.php).

I tried a completely fresh test install in another directory and it worked perfectly. But as soon as I moved the old database into the test site (after changing the 2 urls under wp_options), it has that error again and I can’t post comments. So I’m sure the problem must be somewhere in the database, but I don’t know what I should be looking for. Does anyone have any ideas?

This is the currently broken test site-
http://rahenna.com/loltest/

And the currently broken real site-
http://rahenna.com/journal/