actually it was not the first time the upgrade failed on this board:
- upgrade 5.11 to 5.15 -> returned successfull
- reboot
- at boot system can not find file: sun6i-a31s-bananapi-m2.dtb
(file sun6i-a31s-sinovoip-bpi-m2.dtb for BPi M2 is available)
- fixed it by linking to the existing device tree blob sun6i-a31s-sinovoip-bpi-m2.dtb
- reboot again
- no network
(my /etc/network/interfaces has been set to armbian default, same as /etc/network/interfaces.default, this happened the first time)
fixed it again by setting the correct content
I don't know if the upgrade process is the problem or if there is something messed up on my board...
Now everything is working again... But I guess it will be the same next time...