Magento changed the way credit card numbers are validated in Magento version 126.96.36.199. The new method requires an additional JS include in your page.xml.
Checkout the thread at the following link for detailed instructions on how to fix this issue..
BE SURE TO FLUSH YOUR CACHE AFTER IMPLEMENTING THIS FIX.
If you are still having problems after implementing this fix, try disabling Mage_Centinel in the magento admin at configuration => system => advanced....
Hope this helps...