Hi everyone,
I did some testing in the past days (no UART boot logs though):
Deelan suggested that the boot problem may stem from uSD card due to the voltage error message. Hence, I have made two things:
Made an image of the current Armbian Fossa install (which is btw on a Kingston 32 GB uSD) and wrote it to a Samsung 32 GB uSD. Behavior did not change.
I made a fresh install of Armbian Focal and I chose for an older version 21.08.1: this release from 26-Aug-2021. In short, it, of course, booted up without any issues. I could soft reboot it several time without any issues... the I ran apt update && apt upgrade. The system was brought to the current state (i.e. 21.08.6). The system rebooted ad came online just fine after system/kernel update. All following attempts to reboot the system led to the dark silence. For the note, I have tested it with and without HDDs just in case. The result was the same.
For the sake of testing I installed Armbian Bullseye 21.08.1 on the Samsung uSD. The result was the same as for fresh install of Armbian Fossa: 21.08.1 booted up and I soft rebooted it about 10 times with no issues, when the system was updated to 21.08.6, it came online from a soft reboot the first time after update, any follow up attempt of a soft reboot led to dark silence and the system could be brought back online only by unplugging the system.
In summary, it looks like Cornelius is right and some of the patches, which were submitted and accepted in the meantime, rendered soft boot of the board/amlogic SoC impossible. I guess I have to cope with it as it is now and shutdown/cold boot the system after each kernel update. Hope that someone with knowledge of this business will get down to the core of the problems (e.g. revert some of the late changes).
If there is anything that I can test further.