Hello,
What kind of system? amd64 and i386 use the official plex repo. So, if you aren't seeing an update, then there isn't one.....
Hmm, but this way is not going 
If you used official plex repo:
deb https://downloads.plex.tv/repo/deb/ public main
so you get this Message:
Holen:14 https://downloads.plex.tv/repo/deb public InRelease [3.408 B]
Ign:15 https://dl.bintray.com/openmediavault-plugin-developers/arrakis stretch InRelease
Ign:16 https://dl.bintray.com/openmediavault-plugin-developers/arrakis-plex stretch InRelease
Holen:17 https://dl.bintray.com/openmediavault-plugin-developers/arrakis stretch Release [4.389 B]
OK:17 https://dl.bintray.com/openmediavault-plugin-developers/arrakis stretch Release
Holen:18 https://dl.bintray.com/openmediavault-plugin-developers/arrakis-plex stretch Release [3.540 B]
OK:18 https://dl.bintray.com/openmediavault-plugin-developers/arrakis-plex stretch Release
OK:19 http://cdn-fastly.deb.debian.org/debian stretch-backports InRelease
Holen:21 https://downloads.plex.tv/repo/deb public/main amd64 Packages [5.409 B]
Es wurden 8.817 B in 0 s geholt (12,0 kB/s).
Paketlisten werden gelesen... Fertig
W: Konflikt bei Distribution: https://downloads.plex.tv/repo/deb public InRelease (public erwartet, aber bekommen)
Alles anzeigen
And nothing wil be updated 
So the Repo ist "public main" and now message missmatch with "public InRelease" 
the same Massage will get you ift you edit your "nano omv-extras-org.list"
# This is file is generated.
# OMV-Extras repos
#
# OMV-Extras.org
#deb https://dl.bintray.com/openmediavault-plugin-developers/arrakis stretch main
# Plexmediaserver
#deb https://dl.bintray.com/openmediavault-plugin-developers/arrakis-plex stretch main
#deb https://downloads.plex.tv/repo/deb/ ./public main
deb https://downloads.plex.tv/repo/deb/ public main
Alles anzeigen
So the official repo "deb https://downloads.plex.tv/repo/deb/ public main" can not used, update this manuel with:
wget https://downloads.plex.tv/plex-media-server-new/1.15.3.876-ad6e39743/debian/plexmediaserver_1.15.3.876-ad6e39743_amd64.deb
apt install ./plexmediaserver_1.15.3.876-ad6e39743_amd64.deb
rm plexmediaserver_1.15.3.876-ad6e39743_amd64.deb
for the other Links to the newest Plexmedia you can see (select) it in this Link: https://www.plex.tv/de/media-server-downloads/
On my System it works fine with update Manuel:
root@OMV-NAS-i7-4770:/etc/apt/sources.list.d# screenfetch
_,met$$$$$gg. root@OMV-NAS-i7-4770
,g$$$$$$$$$$$$$$$P. OS: Debian 9.8 stretch
,g$$P"" """Y$$.". Kernel: x86_64 Linux 4.19.0-0.bpo.2-amd64
,$$P' `$$$. Uptime: 23h 44m
',$$P ,ggs. `$$b: Packages: 677
`d$$' ,$P"' . $$$ Shell: bash 4.4.12
$$P d$' , $$P CPU: Intel Core i7-4770 CPU @ 3.9GHz
$$: $$. - ,d$$' RAM: 614MiB / 7894MiB
$$\; Y$b._ _,d$P'
Y$$. `.`"Y$$$$P"'
`$$b "-.__
`Y$$
`Y$$.
`$$b.
`Y$$b.
`"Y$b._
`""""
root@OMV-NAS-i7-4770:/etc/apt/sources.list.d#
Alles anzeigen
Maybe, everything is wrong by plex or OMV4
i found this posting with my writing Problem in Plexforum:
https://forums.plex.tv/t/how-t…rver-using-apt-get/238304
https://support.plex.tv/articl…nux-server-distributions/