Setting innodb_default_row_format globally is apparently not yet supported in my MariaDB version.
mysql -u root -p Enter password: Welcome to the MariaDB monitor. Commands end with ; or \g. Your MariaDB connection id is 114836 Server version: 10.0.38-MariaDB-0ubuntu0.16.04.1 Ubuntu 16.04 Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. MariaDB [(none)]> SET GLOBAL innodb_default_row_format=DYNAMIC; ERROR 1193 (HY000): Unknown system variable 'innodb_default_row_format'
But I have to do a MariaDB update to at least version 10.2 soon anyway, because the new major version 21 of Nextcloud requires at least this version.