It seems to be intermittent? I tested with a credit card (after a customer reported the same error) around 10 or 11, and it gave me the error. After returning from lunch around noon I saw that an order had come through successfully so I tested again (with the same credit card) and it worked fine.
I’m still curious about what caused the issue in the first place. We hadn’t made any major changes on our end before OR after experiencing the issue.