M.2 NVMe slot is also WORKING. At least the SSD gets detected with "cat /proc/partitions". It is not listed under "df -h" but I think this is because it is not mounted yet...

Okay. Thank you!

Since you have now confirmed the HDMI functionality, the time has come for me to retire further support. I am only interested in generic support; for Armbian-specific issues, you'll have to wait for others who are interested.

Okay, one step further: HDMI output is WORKING! initially I had the M1S connected to the Aux input of my AV receiver. There seems to be a problem with display detection (EDID, available modes) and so the M1S does not know which display mode to set. I then connected the M1S directly to an HDMI input of the TV and nothing changed. Rebooted and ... Surprise! Video output works, EDID information and available modes are available (wasn't when connected to AV receiver). So how can I tell Armbian to force a specific display mode even when no screen information is available?

To be honest I currently don't know. I booted my first from SD card. That worked as described above. After that, I've shut down the M1S, set the M1S to UMS mode and cloned the SD card to the internal eMMC. Booted as described. But please tell me how to perform that test and I can give your kernel build a try . Greetings