Here was a user with my box (KM9 Pro Classic) and downgrade to AndroidTV 9:
https://discourse.coreelec.org/t/ce-not-booting-on-updated-firmware-android-10-mecool-km3-atv/11723/17?u=cenew
However, the post is almost a year old and the link there no longer works (Not Found). Maybe I should ask there?
I don’t know anything about that. I first have to find out what exactly I have to do.