Doing a bit of digging online revealed this
article published just yesterday, which revealed apparently a player on Reddit contacted SE's support line and the SE rep acknowledged that they are looking into the issue.
According to that article, it appears to be a problem with the new payment vendor blocking certain types of payment cards which they shouldn't be (both Visa debit and credit cards, a 'Discover' card, and apparently Mastercards issued by a certain UK bank), so if true, means that hopefully a fix is in the future, and that this is an actual fault and not by design. But I have no idea at how accurate this article is.
So all we can do is sit tight and wait I guess.