The database location as per gui doesn't appear to affect the Plex config. It's starting in the /var/lib/plexmediaserver insted of /myvolume/plexmediaserver.
As for now i just made a Symlink from /myvolume/plexmediaserver to /var/lib/plexmediaserver and it now uses the correct Plex config.
Is it a bug in the Plex plugin?
Output of dpkg -l | grep openm
Code
ii openmediavault 3.0.59 all Open network attached storage solution
ii openmediavault-couchpotato 3.2.2 all OpenMediaVault CouchPotatoServer plugin
ii openmediavault-keyring 1.0 all GnuPG archive keys of the OpenMediaVault archive
ii openmediavault-letsencrypt 3.2 all Generate free and valid SSL certificates for OMV
ii openmediavault-omvextrasorg 3.4.15 all OMV-Extras.org Package Repositories for OpenMediaVau lt
ii openmediavault-plexmediaserver 3.7 all OpenMediaVault Plex Media Server plugin
ii openmediavault-sonarr 3.2 all OpenMediaVault Sonarr plugin
ii openmediavault-transmissionbt 3.0.9 all OpenMediaVault Transmission (BitTorrent client) plug in.