I have solved the problem, or at least the problem is solved. Maybe I had nothing to do with solving it. But call off the developers. I tried again today, and today I was able to register. I don't think I did anything different, so maybe my computer was on vacation. On the other hand, maybe I did do something different.
I will say that it is not intuitive which box one should try to paste into, the large upper one or the small lower one. Once a key is pasted, it is clear where to enter data. But for the first entry it could be labeled "enter code here."