Nokia: I just looked through maybe 20 posts concerning serial number problems and not a single one contains a solution, except for the suggestion to copy & paste.
In this post there is not just complaining. No, I'll describe where I went wrong so that it may help others:
When I got to the page where you can request a serial number there were about 40 products listed. I chose the wrong one and it didn't work of course. I found that the setup program lists the exact product name in its title bar and that's how I found the right product on the request page.