At the moment I am unable to make any hotel bookings via Aegean's website (a wrapper for booking.com with referral tracking).

It fails on the payment stage. Whatever credit card I put in it is rejected with the message: "Sorry, we have not been able to take your payment. Please try using a different credit card or PayPal."

The problem is I've tried over half a dozen different credit cards, and they are all rejected. I've closed the browser, logged out and logged in again, switched to incognito etc., and none of it makes any difference. I simply can't complete any booking because the payment card is always rejected.
I've never experienced this problem before. Is anyone else having similar issues?