This happens occasionally and has been going on for a year or two. It always sorts itself out within a week or so.

It's probably either due to our computers having updated themeselves or (more likely) ebay meddling with their software, making something incompatible.

I just message the seller to explain and apologise for a possible delay in payment due to an ebay glitch, and try again every day until it works.

Just another of the many annoyances that make up the "customer experience" of using ebay these days...