Good news! I was finally able to renew online just now with no error messages and the usual security checks working perfectly.
I don’t know if using my home WiFi rather than the hotel WiFi I was on the last few days made the difference. Given my attempt to renew by phone a few days ago also didn’t work, I suspect not. Most likely the glitch has been resolved by the payment provider. I still have to load the renewal to my Oyster but hopefully there will be no further issues!