OScommerce not passing payment data to payment gateway

My OScommerce webstore has suddenly stopped processing credit card payments for some unknown reason. I have spoken to my payment gateway company 'Realex' and they tell me that they are not receiving any XML information from my site to process. The only error I receive on the site is 'Credit Card Error!' when clicking confirm payment after entering credit card details. I can accept credit cards by manually entering the details on my payment gateway admin page directly so the credit card details and payment gateway side appear ok. Possible suggestions I have be given regarding this are directory security issue, CGI issue, CURL directory issue. If you has solid experience is OScommerce and payment gateways using XML and think you can solve this issue please place your bid.