Suggest you move your files to a location other than /volume1/video. From the Installation & Setup of Plex Media Server on Synology support document.
We urge you to NEVER USE the predefined video, music, or photo shares that Synology created. These shares are owned by Synology’s Video Station, Audio Station, and Photo Station products.
Starting with DSM 7, Plex now runs under the system internal user (not “regular” user) PlexMediaServer. Make sure it has at least read only access to the files.
Edit: See FAQ 30 in ChuckPA’s post.
