Fantastic, this did it. Now the normal KOReader (monthly release) works without a problem, and I have re-registered my Kindle (after which I switched back to airplane mode). Nothing tried to interfere or install a newer firmware version
The one thing I am missing is librarian sync. I understand the the needed python package hasn't been updated. Does that mean that I have to wait until a new version is released, or can I install the version from January?
Thanks again, Marek.
Cheers, Joachim