Thank you, Kirill and Joss for posting here.
Yes, this type of issue is caused by unapproved changes to front-end templates of credit card form. This is a security measure in X-Payments and all you need to do is to approve the changes in the X-Payments admin back-end.
See
http://help.x-cart.com/index.php?title=X-Payments:Troubleshooting#Error_message_at_checkout _instead_of_credit_card_form for more details.