Thanks for the suggestions, and for reproducing the problem. I emailed nokia support, and they responded with the suggestion to clear the browser history (done through the settings app), and to then restart the phone.

After I did this, the store app seems to be working OK now. I haven't tested it extensively, but I was able to complete the purchase I was trying to make originally (by credit card this time!)

It's still a nasty bug. I suppose I could try the bill option again to see whether it might have been server related, as was suggested, but I might leave that for another time.