Maybe I can help.

My url is clean but I have the same problem. After payment, the user is redirected back to the shopping cart.

I disable Calculate shipping cost and anonymous checkout.

1) I set in user management > user settings > Only site administrators can create new user accounts.
and the problem was fixed. I can see the complete page!

2) if I set Visitors can create accounts and no administrator approval is required I'll see this problem again. Require e-mail verification when a visitor creates an account - enable.

3) Visitors can create accounts but administrator approval is required. - no problem.

4) New user registration by invitation only. - I don't test it.

Drupal, last ver of Ubercart and I have i18n module. so my urls looks like
http://site/en/admin/user/settings
http://site/en/catalog

unfortunately my site settings is Visitors can create accounts and no administrator approval is required...