Forum Upgraded from 3.8 to 4.0, unable to post/edit/etc
 

I recently upgraded from vBulletin 3.8 to 4.0. Everything looked great afterwards, but when I tried to make a post, I discovered that something was wrong. I can't "do" anything from the front-end, such as writing threads or posts, or pinning topics, etc. When I try to edit/write a post and hit Submit, it gives me a completely blank page (nothing in View Source, even though it returned 200 OK). If I try to do something from the "Moderation tools" dropdown (on a thread list or inside a thread), it tells me "Invalid action". I don't have any problems just looking at the forums, though. :p

I can do everything in the back-end (AdminCP), but that doesn't do my users any good. The site worked fine before the upgrade. If I upload the forum in a separate directory and create a new database, everything works, but losing the whole database isn't an option. I've even tried using a backup (from 5 days prior to the upgrade) and migrating from there, but I still have the same issue. And I've gone through the list in Maintenence -> Suspect File Versions, and it doesn't seem to notice anything particular odd.

Code:
# php -v
PHP 5.2.0-8+etch11 (cli) (built: May 10 2008 10:46:24)
Copyright (c) 1997-2006 The PHP Group
Zend Engine v2.2.0, Copyright (c) 1998-2006 Zend Technologies

# mysql --version
mysql Ver 14.12 Distrib 5.0.32, for pc-linux-gnu (i486) using readline 5.2
I have absolutely no idea what's going on at this point. Any help would be appreciated!