This has happened to me several times, and it was always because there was a problem with the credit card. In my case, it was because my browser auto-populated it and it was missing a digit, but it's possible there was some other kind of error or problem with it. Their website seems to do that when everything isn't just perfect at checkout, so double and triple check everything... I would be willing to bet there's some tiny little thing that's off and their site doesn't know how to handle it.