So I just tried it myself (sorry for the long delay)
After completing the payment, the user gets taken back to: https://<mydomain>/index.php?option=com_eventbooking&view=complete&Itemid=1795
That error should not being displayed after registration completed like that. Please submit a support ticket sending us super user account of the site and link to the page which I can perform registration and see that error. I will check to see why the issue happens and get it sorted