As of right now, we are unsure of the cause of this error, but have had it occur on about 10% of upgrades so far. Here are the steps to get you back live again:
1. Go to Joomla.org and download the full package of 3.2.4:
http://joomlacode.org/gf/project/joomla/frs/?action=FrsReleaseBrowse&frs_package_id=6882
2. Using FTP, upload all of package, except the /installation/ folder
3. Once done, go to your /cache/ folder and delete all files except index.html
4. Go to phpmyadmin and delete all rows in SESSIONS table (example, DELETE FROM `jos_session`
