There was a bug introduced in MariaDB 10.2.35 and 10.3.26 which breaks connectivity from PHP versions prior to 7.3.

Any sites running PHP 7.0-7.2 would receive the error:

Critical Error Could not connect to the database.

The resolve this upgrade MariaDB to the latest version, or switch to PHP 7.3 or 7.4.

For details on this bug please see: https://jira.mariadb.org/browse/MDEV-24121