Possibly related to the recent update: https://mariadb.com/kb/en/mariadb-10235-release-notes/
Sites running PHP 5.6 will see this error:
Sites running PHP 7.0, 7.1, 7.2 will see this error:
Setting the sites to use PHP 7.3, solves the issue. So the recent MariaDB update appears to have introduced a DB connection issue for PHP =< 7.2.
Another solution is to downgrade. This is also mentioned here: https://xenforo.com/community/threads/mariadb-10-3-26-1-breaks-php-7-2.187331/