Thanks for the hint dcinoz! 
I've already read about this issue, but I haven't yet switched to Bookworm (afaik you have to manually edit the repos of your Mobian system before updating, I wanted to try this as soon as I have a stable backup phone...). At the moment my new/second PinePhone uses the same 'old' software than my first PinePhone, so this known brightness problem should not be the reason.
SSH is not yet set up, but I can try this as soon as the phone boots (I hope it WILL boot (as it did yesterday) when I get home later and turn it on, after it was off for nearly a day...).
I've already read about this issue, but I haven't yet switched to Bookworm (afaik you have to manually edit the repos of your Mobian system before updating, I wanted to try this as soon as I have a stable backup phone...). At the moment my new/second PinePhone uses the same 'old' software than my first PinePhone, so this known brightness problem should not be the reason.
SSH is not yet set up, but I can try this as soon as the phone boots (I hope it WILL boot (as it did yesterday) when I get home later and turn it on, after it was off for nearly a day...).
Using encrypted Mobian (bookworm) on pmOS CE Convergence PinePhone (3GB/32GB) as Daily Driver, and Beta Edition Convergence PinePhone (3GB/32GB) with encrypted Mobian (bookworm) as Backup device.