You defined PGID and PUID. PGID is "users" on OMV. Have the users access to the media files on filesystem level?
Which user has the specified PUID?
problem was solved by going thru OMV web GUI, click shared Folders, then selecting Plex shares, editing the "ACL" of each, and changing recursively the owner to Plex. then re-add libs in plex interface, and rescan libraries.