mariabackup from remote mariadb10.3 galera | '/var/lib/mysql/' (Errcode: 2 "No such file or directory")
Wrong approach - mariabackup isn't designed to work remotely. See also MDEV-15075.

So you need to run mariabackup locally on one of the nodes in the cluster. 

To prevent the backup from slowing down the cluster, you can prevent the node from engaging Flow Control and allow it to fall behind the other nodes by doing: 

SET GLOBAL wsrep_desync=ON;  


Once the backup is completed, you can do:

SET GLOBAL wsrep_desync=OFF;


(See wsrep_desync in the MariaDB knowledge-base.)

Obviously, don't direct any database traffic at the node while you're running the backup.