I also had issues with Plex in the past. I recommend using the Plex image from their official website. I've read somewhere that the omv plugin has multiple issues and will be gone in some time.
Just download the .deb from here , dpkg -i it and you should be fine.
I use Raspberry Pi3 image (OMV_3_0_88_RaspberryPi_2_3_4.9.41.img.xz) and reisntalled
I will try install plexmediaserver-ros6-binaries-annapurna_1.11.3.4803-c40bba82e_armel.deb with dpkg
Hi all!
I updated a plugin via WEB UI
Extracting readynas.deb ...
>>> *************** Error ***************
communication failure
<<< *************************************
rm: cannot remove '/etc/systemd/system/plexmediaserver.service.d/override.conf': No such file or directory
Synchronizing state for plexmediaserver.service with sysvinit using update-rc.d...
Executing /usr/sbin/update-rc.d plexmediaserver defaults
insserv: warning: current start runlevel(s) (empty) of script `plexmediaserver' overrides LSB defaults (2 3 4 5).
insserv: warning: current stop runlevel(s) (0 1 2 3 4 5 6) of script `plexmediaserver' overrides LSB defaults (0 1 6).
Executing /usr/sbin/update-rc.d plexmediaserver disable
insserv: warning: current start runlevel(s) (empty) of script `plexmediaserver' overrides LSB defaults (2 3 4 5).
insserv: warning: current stop runlevel(s) (0 1 2 3 4 5 6) of script `plexmediaserver' overrides LSB defaults (0 1 6).
Synchronizing state for plexmediaserver.service with sysvinit using update-rc.d...
Executing /usr/sbin/update-rc.d plexmediaserver defaults
insserv: warning: current start runlevel(s) (empty) of script `plexmediaserver' overrides LSB defaults (2 3 4 5).
insserv: warning: current stop runlevel(s) (0 1 2 3 4 5 6) of script `plexmediaserver' overrides LSB defaults (0 1 6).
Executing /usr/sbin/update-rc.d plexmediaserver enable
Synchronizing state for plexmediaserver.service with sysvinit using update-rc.d...
Executing /usr/sbin/update-rc.d plexmediaserver defaults
Executing /usr/sbin/update-rc.d plexmediaserver enable
Job for plexmediaserver.service failed. See 'systemctl status plexmediaserver.service' and 'journalctl -xn' for details.
Done ...Checked the /var/lib/plexmediaserver: is empty 
I removed a package (apt-get remove plexmediaserver-installer) and I tried again in CLI but result is agint wrong:
root@ovm:~# apt-get install plexmediaserver-installer
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
avahi-utils (0.6.31-5)
Use 'apt-get autoremove' to remove it.
The following NEW packages will be installed:
plexmediaserver-installer (1.10.1.4602-f54242b6b-1)
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/7526 B of archives.
After this operation, 0 B of additional disk space will be used.
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
LANGUAGE = "en_US.UTF-8",
LC_ALL = (unset),
LC_CTYPE = "UTF-8",
LC_MESSAGES = "en_US.UTF-8",
LANG = "en_US.UTF-8"
are supported and installed on your system.
perl: warning: Falling back to a fallback locale ("en_US.UTF-8").
locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_ALL to default locale: No such file or directory
Selecting previously unselected package plexmediaserver-installer.
(Reading database ... 49840 files and directories currently installed.)
Preparing to unpack .../plexmediaserver-installer_1.10.1.4602-f54242b6b-1_armhf.deb ...
Unpacking plexmediaserver-installer (1.10.1.4602-f54242b6b-1) ...
Processing triggers for systemd (215-17+deb8u7) ...
Setting up plexmediaserver-installer (1.10.1.4602-f54242b6b-1) ...
Downloading readynas package ...
######################################################################## 100.0%
Passed checksum test.
Extracting readynas.deb ...
Synchronizing state for plexmediaserver.service with sysvinit using update-rc.d...
Executing /usr/sbin/update-rc.d plexmediaserver defaults
insserv: warning: current start runlevel(s) (empty) of script `plexmediaserver' overrides LSB defaults (2 3 4 5).
insserv: warning: current stop runlevel(s) (0 1 2 3 4 5 6) of script `plexmediaserver' overrides LSB defaults (0 1 6).
Executing /usr/sbin/update-rc.d plexmediaserver enable
Job for plexmediaserver.service failed. See 'systemctl status plexmediaserver.service' and 'journalctl -xn' for details.
root@ovm:~# systemctl status plexmediaserver.service
● plexmediaserver.service - Plex Media Server for Linux
Loaded: loaded (/lib/systemd/system/plexmediaserver.service; enabled)
Drop-In: /etc/systemd/system/plexmediaserver.service.d
└─db_folder.conf
Active: failed (Result: start-limit) since Tue 2018-02-13 22:45:04 CET; 2min 32s ago
Feb 13 22:44:59 ovm systemd[1]: Failed to load environment files: No such file or directory
Feb 13 22:44:59 ovm systemd[1]: plexmediaserver.service failed to run 'start-pre' task: No such file or directory
Feb 13 22:44:59 ovm systemd[1]: Failed to start Plex Media Server for Linux.
Feb 13 22:44:59 ovm systemd[1]: Unit plexmediaserver.service entered failed state.
Feb 13 22:45:04 ovm systemd[1]: plexmediaserver.service holdoff time over, scheduling restart.
Feb 13 22:45:04 ovm systemd[1]: Stopping Plex Media Server for Linux...
Feb 13 22:45:04 ovm systemd[1]: Starting Plex Media Server for Linux...
Feb 13 22:45:04 ovm systemd[1]: plexmediaserver.service start request repeated too quickly, refusing to start.
Feb 13 22:45:04 ovm systemd[1]: Failed to start Plex Media Server for Linux.
Feb 13 22:45:04 ovm systemd[1]: Unit plexmediaserver.service entered failed state.How can get again a Plex Media Server?
THX!