Unable to checkout

We attempted to upgrade to the new version of Ubercart (1.2) from 1.0. However, after upgrading, we were unable to complete a transaction. We could get to the "Review Order" page, however, upon clicking "Submit Your Order", you are sent back to the "Checkout" page. You can go through this loop as many times as you like; each time it will create a new order (which can be viewed in the Orders page). It suddenly feels a lot like Hotel California. I can checkout, but I can never leave.