It appears something is broken with Plex again?
I just did a fresh Debian 8 install on a spare PC I have (no more RPI, rycoaaron, lol), and manually installed OMV 3.0.64.
Several plugin installed already without issue, but.....
Trying to get the Pex plugin installed, OMV-Extras -> Update ends with this line:
W: Conflicting distribution: downloads.plex.tv public InRelease (expected public but got )
but the entry does appear in the Plugins list as plexmediaserver 3.8.1.
However, try to actually install it and you get the same error as above:
W: Conflicting distribution: downloads.plex.tv public InRelease (expected public but got )