Hi,
after giving it a rest for the night and reading your responses that it should work, I plugged in the Kobo again this morning, removed all books again and copied them back. Now it works.
All (the later) experiments before were done without unplugging the Kobo once, and maybe calibre had a problem with that.
I have a second plugboard for my PRS-T1, which works without problems, and I think (i.e., hope) that it doesn't collide with the Kobo plugboard.
So, thank you very much for your help (even though I do not understand why it works now when it didn't before), and as a hint for others: Unplug the Kobo, restart calibre, and then try again.
Cheers, Joe