Nevermind, I already found the solution!

http://www.thewitcher.com/backup/

I have no idea what the problem was exactly, because the original message was removed. I think it's an annoying practice to edit and remove the original question and not even telling what the solution was. Maybe someone has or will have the same problem and they might find this thread, but there's nothing helpful left here.

Nothing personal, it just irks me when people do this, even when the question seems stupid to them afterwards. The internet has tons of users, someone else probably will have the same question and will google for it.

In case anyone else stumbles through here, I couldn't find my key at first either, but then found it printed inside the manual cover – not printed on the back cover, as one might assume.