Thanks for the details. This piece here ^ looks as if you did not create a symlink, but still have the original folder with all contents at /var/lib/nextcloud (the initial ‘d’ from ‘drwxrwxrwt’ says it’s a directory, not a link which would be ‘l’).
But this does not explain why NC does not find your .ncdata file which probably also exists there, right?
BTW, you should have podman as alternative to the docker command, if that helps in your research. Others have examples how to use it in this thread.
That being said, I am out of my depth here. Maybe someone who runs NC themselves can help?