It's either Serif or their payment partners, I don't know which and frankly don't care. There are people from all over, using different banks. It's not very plausible that several banks across the world are all collectively failing. I bought some stuff on Steam yesterday, same card, same bank and payment went through fine, and I got my stuff.
Serif seems to be doing absolutely nothing about this. "It's a small subset of users" well then guess we'll just go to hell then? How would Serif feel about handing out the license to this "small subset" of users, after all it can't be too big of a loss since it's such a small number of people and doesn't warrant any response or immediate action...
Having a problem is one thing; having no meaningful response or public statement to address the issues, no replies from support and only excuses is just incompetent.