[Ulk Bäär]

Thank you for the explanation.

I booted with

cmdline.txt Wrote:rootflags=subvol=/@,autodefrag,compress=lzo

and

your proposal Wrote:rootflags=subvol=.,autodefrag,compress=lzo

and

cmdline.default Wrote:rootflags=subvol=root/@,autodefrag,compress=lzo

in cmdline.txt
Some of those also wuthout ,autodefrag,compress=lzo .

Always black screen with only an underscore in the top left (not blinking) and only the red LED of the RPI lit.
Using the files provided in post #18 I get a complete black screen.