I just had the same problem, had to book a flight in emergency (the flight leaves in about 11 hours, and the phone lines are not open right now).

I tried Chrome, Firefox, different computers and different payment cards and th payment still failed.
Finally I tried with Internet explorer with the card that I used first, and logging in to my session on their site, and the payment just went through.

I really don't understand why all of this is even an issue, but I am glad that it got sorted in time.