Comment # 8 on bug 1166781 from Kristyna Streitova

It really looks like you hit an upstream bug (MDEV-21244). Let's wait if
upstream fix it in the next release.

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Btw. This bug initially looked like it was caused by the authentication changes
in MariaDB 10.4 that cause that (simplified) one has to run "mysql" command as
a root or sudo from now on. I'm just adding a note about it here because it can
be a solution for some users.

"Two all-privilege accounts were created. One is root@localhost, it has no
password, but you need to be system 'root' user to connect. Use, for example,
sudo mysql. The second is mysql@localhost, it has no password either, but you
need to be the system 'mysql' user to connect. After connecting you can set the
password, if you would need to be able to connect as any of these users with a
password and without sudo."

Please see https://mariadb.com/kb/en/authentication-from-mariadb-104 for more
details about authentication changes.