Receiving this mariadb error only when using LuckPerms

[06:39:49 WARN]: [Craft Scheduler Thread - 686] WARN com.zaxxer.hikari.pool.PoolBase - luckperms - Failed to validate connection org.│··············································································
mariadb.jdbc.MariaDbConnection@6ec8ad85 (Connection.setNetworkTimeout cannot be called on a closed connection)                       │··············································································

Can you please verify that you entered valid MariaDB authentification data?
Also when does this error happen? During startup or does it happen again and again?

Yes, it is the same information I have used for all other plugins that connect to the MariaDB database. This occurs every time there is an update to someone's permissions.

Also shows when a player joins the server.

You might wish to tweak some of the connection pooling settings in the LP config, especially the ones nearer the end.
https://github.com/lucko/LuckPerms/blob/master/bukkit/src/main/resources/config.yml#L299-L326

LuckPerms delegates management of MySQL connections to HikariCP. You can see a bit more info about each of these options on their page here: https://github.com/brettwooldridge/HikariCP

The default options in the LP config are designed to work (and do work!) for the vast majority of users - however if you or your host have made changes to the SQL server config, then sometimes there are issues.