This happens to me a lot when I am donating, I have not found a fix for it other than wait a day or two then try again, preferably not just after someone else has donated, hope this gets sorted for you
The response I got on the ticket was: "This means that the provider rejected the payment. This can be caused by many possible reasons (for example, if the payer's card was issued on a different country than his current location), unfortunately since this happens externally we are not provided with information on each case." I have re-added Paypal, so you could use that if it would be better.