So I removed SD Card with roms and media and put a Batocera v35 SD Card in my Odroid.
Turned the MMC boot off with the selector at the front.
Ran Batocera, and installed Batocera on the MMC via the menu options.
But the installation files didn’t came from the SD Card (v35), but from internet, so I have again v36 on the MMC.
After the install, swapped the SD cards to get my collection back, put the switch back to boot from MMC and now, it’s running just fine! Even the CPU speed is set on “High” which is probably a left-over from the settings I had from the v35 on the SD Card.
Anyway: everything is running fine now.
The problem with these things: you set it up once, being well informed by YT videos and sites, but when it works, you’ll easily forget all the details as no-one is daily working with these hardware devices or operating systems.
So it’s quite difficult to remember how it works in details.