no, i made no hardware related changes. I just made updates from within Libreelec GUI, for libreelec itself and the firmwares for example.
Now i try to find out how to get the profile and network share settings from the broken system to rebuild from a new installation.
At the moment the broken system is untouched.
I changed the hdmi output from 2 to 1 on the raspi4 (counted from the power plug) and started the System again.
After a while i switched the TV on and...
Kodi 9.2.3 was running!!!
Thanks anyway for all ppl trying to help!