Good evening, the problem persists, but I can start the database with sudo /usr/sbin/mysqld --skip-grant-tables --general-log
and afterwards execute mysql_upgrade without it throwing an error. The downside is that it does not change anything. I still cannot install mariadb-server-10.1 without the error I metioned. Would it help to purge the package completly and reinstall the tables from the backup? Harald
