I've had problems with X-payments as well. First make sure you follow the steps in:
http://help.x-cart.com/index.php?title=X-Payments:Installation
Next, we ran into a problem with our hosting and SSL. Are you hosting x-cart/x-payments yourself or using a service (such as EWD Hosting or Total Server Solutions)
If so, have them check your problem, as they seemed to help us very easily and recognize exactly what is going on.