Yesterday I finally installed a RAM stick that seems to be working (a 16GB Hynix of the type listed at the Raptor Wiki). It seems that it works in the A slot instead of the B which is recommended in the manual. For my great surprise the system finally started up to Petitboot. So I connected a USB stick with the Fedora 30 and tried to boot. It worked but installed just a base server system where the only configured package repository was that USB stick. I have not used RedHat/Fedora from the RedHat Linux 6.2 so I was a bit helpless here. Thus I tried to boot a Void Linux CD. It booted and it seems to be OK (well, I should read the manual first as it claims to be very different from mainstream Linux distros).
At the moment I decided to use the Fedora (which seems to be well supported here until I will sure that the whole computer works well. So I reinstalled the Fedora once more and this time with online package repository configured. I have not selected the Fedora Workstation but a system with simple window managers and I also have added some desktop environments (the MATE and the GNOME).
The installation required some time and then it ended in a working system with X11. The MATE is fast and the GNOME is fast, too (on the X11 – I still have not installed the discrete graphics so I don't want to try the Wayland).
There are no screenshots so far, sorry. I still dind’t modified the xorg.conf so my screen is still just 1024×768 so it does not look promising.