Ubuntu 20.04 will come with systemd 244 which by default set fs.protected_regular = 1 which prevents to start MariaDB 10.3 Galera cluster, which, on the other hand, is selected for Ubuntu 20.04. As a workaround, it can be possible to set fs.protected_regular to 0, but this is a system-wide security feature.
The fix for MariaDB 10.4 and 10.5 is available - https://jira.mariadb.org/browse/MDEV-21140
In order to make MariaDB 10.3 compatible with default security settings of Ubuntu 20, the
MDEV-21140 need to be ported to 10.3.