I'm back in business after going back to "image: webhippie/mariadb:latest-arm32v6" and a restore of the last but one backup.
I had to go this way because I didn't succeed with the latest mariadb image from webhippie which is release 10.6. I found several links where an incompatibility between nextcloud and mariadb 10.6 are described, e.g. https://github.com/nextcloud/docker/issues/1492
I'll stay for a while with mariadb:latest-arm32v6 until the incompatibility will be resolved, instead of to try the workaround mentioned in the line above.
linuxserver/mariadb is at 10.5 → No incompatibility