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:
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`)
When we encountered this error, the final result was it was caused by an outdated version of the JPayPlans component.