CiviCRM 4.6.3 and Drupal 7.38. I have gotten past credit card and CVV checks, verified info on Confirmation page, submitted the event registration, but browser comes back with "Page Not Found - The requested page '/civicrm/event/register' could not be found." When I check the CiviEvent Dashboard I see that the individual and any guest have been properly registered. CiviEvent is just not finding/displaying the Thank You page. That address seems strange. As a test I set up a CiviContribution page and ran through a contribution without a hitch. So I don't understand what is happening with CiviEvent. Ideas?
please try to update civicrm to latest version - I did made a quick test and it works correctly displaying the Thank you page
Even after updating, if you are getting the error message then please let me know the payment process which you are using so i will give a quick test