Thank you for updating with the solution!
This was the first thing I tried, but oddly it didn't work. So I followed the error code recommendations and enabled javascript, then tried a different browser, then from advice on another post I saw I disabled adblock. It still didn't work. Then found this post and thought might as well try deleting and re-entering payment details again, and voila! Not sure which combination of things did the trick but if anyone is stuck it can't hurt to try them all.