I found the root cause of the problem!
The installation of my system was migrated from owncloud to nextcloud following this description.
The origin installation was performed using apt-get on a raspi3 box.
The owncloud installation package was not removed using the package manager. Yesterday evening a newer owncloud (9.1.2) package was amongst several other packages and was extracted into the 10.0.1 nextcloud directory structure.
Solution: 1. remove the owncloud package with package manager
2. download and extract nextcloud into the owncloud directory structure
kind regards
Peter