Can't Review Order, button loops back to checkout form '/cart/checkout' I'm having a problem which is prevent me from reviewing my order after completing the checkout form. Once I click on "Review Order" the page essentially refreshes itself because of the loop. Others with the same issue have varying solutions, I've tried a handful so far without any luck. 

It seems the issue may be with the session being lost somehow, but I'm not sure what could be causing this. I haven't changed anything related to ubercart in months, yet the problem appeared days ago. Any suggestions or work arounds would be greatly appreciated.

I'm using PayPal Website Payments Standard, Drupal 6.2, Ubercart 6.29. 

Thanks.
Thanks to David's response, I was inspired to investigate my upgrade from uc 2.8 -> 2.9 and ran the update.php script. It seems it was not run at the time of the upgrade, so now that it has the problem is solved.