Also, I've just tried to place a test order as "Alex Tester" and I've got an error message:
The payment processing system is temporary unavailable due to an internal error.
What do you see in X-Payments logs?
See
http://help.x-cart.com/index.php?title=X-Payments:FAQ#Where_can_I_find_X-Payments_logs.3F to understand how to find them.