After noticing a failed payment in a client’s AdWords account we contacted American Express to see what the issue was. Upon speaking to American Express they informed us of an email they received regarding a merchant processing issue between Google and American Express. We promptly added a third backup card, a MasterCard, as our primary and backup are both American Express cards.
We then contacted Google AdWords support Billing team and were told there has been “more than one” incidence of American Express processing issues over the past year. The proper procedure is to re-enable the form of payment and/or re-enter your American Express card details into AdWords and the issue should resolve itself.