Hi Everyone,
I found an old thread about exact the same issue with Auth.net module in an old X-Payments version. You must be using v1.0.5 - if so, upgrade your X-Payments to v1.0.6 or v2.x.
Users of downloadable X-Payments can find latest versions in their File Area (for accessing v2.x file an upgrade fee is required - see why to upgrade to v2 at
http://www.x-cart.com/x-payments-feature-tour.html ).
Users of hosted X-Payments - send us a ticket for a free upgrade to the latest version.