This was the solution!
Now the EOS-Arm installation starts and can be finished.
Very interesting are following things:
The commandline is needed after the installation too.
Using the parameter video=HDMI-A-1:1920x1080@60 does not work.
PlasmaLoginManager does not start with full HD but with 1366x768 even if the monitor is full HD.
In Plasma/KDE it is no problem to change the setting and after logging in the resolution changes to full HD.
Is there a possibility to change the edid.dat to use full HD at the startup, or better how can I create another edid.dat on my own? I can see during the startup messages, that the resolution switches back from full HD to 1366x768.
If it was me I would try to locate some firmware designed for your monitor and put it under where it is supposed to be some where under /usr/lib/firmware. I would guess it was at some time in the linux-firmware package and may have been removed at some point. Right now it appears to be trying to use some generic form of firmware which is not cutting it.
From my experience the first boot uses config’s in config.txt for your monitor and the config in cmdline.txt is used when when graphics / mesa takes over when it goes in graphics mode.
My edid.dat is just a band aid. Your system was able to make things work some what but needs full info intended for your monitor. One needs to be booted into a working system with the monitor booted using it’s firmware to generate a edid.dat. I have in the past was able to boot in fkmsv3d and get edid info and use it to boot kmsv3d. A TV’s native resolution of 1366x768 can be problematic for linux devices.