Also, in order to find place of the trouble try to do this - create a free trial account of X-Payments Hosted at
https://www.x-payments.com/trial-demo.html and connect your X-Cart store to it instead of your self-hosted copy of X-Payments. If troubles disappear after that - a trouble is definitely sitting in your self-hosted installation of X-Payments. If not, I highly recommend you to work with our techs in your HelpDesk account. Nobody else reports such troubles.