I'm just guessing here. But it looks like "Response is not valid" means that the response that X-Cart is getting from the processor is not what it expects. I know this doesn't solve the problem, but what I can say with authority is: It is likely that the processor has changed something on their side, and X-Cart needs to be updated to reflect and properly interpret this change. I would open a ticket with XC, or contact a developer to handle this issue. (like me)
The dev should communicate with the payment processor, and determine what is wrong. The good news is, the payment processor has a vested interest in figuring this out, they can't make money unless orders go through. So if you are a dev working on something like this, remember that you can't be on both sides of the issue, you have to rely on the payment processor support.