I just remember something that I DID do and could probably cause this..

in the /catalog/includes/modules/payment/
folder, I moved all the modules, except purchaseorder.php and plugnpay_api.php
to /catalog/includes/modules/payment/other
of course I left the folders alone, I just moved the php files except those two.

Sorry, I had just remembered.. Now, I've moved cod.php and moneyorder.php back to the modules/payment/ folder.

Do you think that by moving those files to another directory it caused this error?

I can't really test at the moment... I only have one card and the cart is telling me I've used the same card too many times (6 times x.x) than allowed in the time permitted.