@gothrek Hey, I'll try to explain my problem in English as best as I can:

My first problem was about the fba_libretro rom set-version, 'cause in the readme-file it's written that you've to use the version 0.2.97.37, in contrast to the german translated recalbox manual where it is written, that you need the rom set-version 0.2.97.38 since Recalbox 3.3.0. This case is already solved, the german manual is just out of date I guess.

The second problem is, that one of the rom sets, that I would like to use with fba_libretro doesn't work (in contrast to two other rom sets I'm using with fba_libretro). It's the rom set of Demon Front and I have the 0.2.97.37-version of the v105-set as well as the pgm.zip (also the 0.2.97.37-version) in the fba_libretro-rom folder, but when I start Demon Front in the Emulation Station, it will only show the Polygame Masters-intro until it displays a date and a time. After that, I get back to the ES-Menu automatically. I've the same problem with the Demon Front-rom sets v102, v103 & v106, and I still don't have a solution for that.