The answer by Thomas is correct, but get's reset by updates every few months. So here is a permanent solution:

Use systemctl edit mariadb to create a file overwritting the default settings of the mariadb service. (In debian it's located in /etc/systemd/system/mariadb.service.d/override.conf)

Set the same setting Thomas changed in the file:

[Service]
ProtectHome=false

Use systemctl daemon-reload to reload the systemctl config.

参照：https://stackoverflow.com/questions/38529205/mariadb-cannot-start-after-update-warning-cant-create-test-file-home-mysql