There will be an upgrade later today to version 3.5. User's that had previously installed are going to have to follow these instructions. Do not upgrade when you see that the upgrade is available because you need to reinstall.
apt-get purge openmediavault-plexmediaserver
apt-get purge plexmediaserver
rm -rf /etc/systemd/system/plexmediaserver.service.d
Then go to the plugin section and install the plugin again.
Changelog done... please build and get into repos. Just the plugin for OMV 3.15+.