Uninstall the DSM 7 Plex app with ERASE (in red)
– We’re going to get rid of it and then get your DSM 6 Plex upgraded
Using this FAQ (27) – Step: Scenario 2
– Use it as the guide to give SYSTEM INTERNAL USER PlexMediaServer “Full Control” (Admin,Read,Write permission) of files and folders of the “Plex” shared folder.