Thank you. But since i had no success randomly trying the solution to other peoples problems I took the arduous approach, installed a clean Armbian Bullseye, used the NPC-curl-Installer, manually recreated all user accounts and coppied an rescanned their files.
Now I am still trying to rescue my family calendar from the remains of the old database, to feed them into Calcardbackup, but that is a minor issue.
My mistake was, that I had tried too hard last winter to update my system using an image, that other users already had complained about. That went a bit dodgy and I was on borrowed time from then onwards. Since I can hardly expect the NCP maintainers to keep current images for all kinds of exotic SBCs of the last decade, I have learned my lesson: Stick to the Armbian guys for best HW support and current OS and then add NCP on top.
UPDATE: Also I learned to keep the automatic NCP- and NC-updates deactivated and apply those manually after a fresh backup and some research about common issues.