In my last post I had to get a bit acquainted with DietPI.
I was thinking there wasn't a normal user so I added user "pi" now.
I overlooked that a normal user "dietpi" already exists.
So we user "dietpi" is usable in the future.
Anyway, I did somewhat a new RetroPie-Setup as user "dietpi" again.
Everything seems to work ok now and Emulationstation works too.
The desktop parts at the right and the left when starting a libretro core, that I described in my earlier post, are gone now too.
The experience is just like the normal "Debian 11 Bullseye / 32 bit" with mate desktop installed.
After making a fresh OS you have to :
Too bad, I have no sound from hdmi, it seems some audio packages could be missing ;-(
Jack output works though. ;-)