You are running DSM 7.
Plex now runs under the system internal user PlexMediaServer, not Plex.
You need to reset your file permissions to give PlexMediaServer access to your media folders.
See DSM 7 files not available for instructions.
You may also wish to review DSM 7 - Plex Media Server - Release Info & Migration instructions . It provides an overview of the changes when running Plex on a DSM 7 system vs a DSM 6 system.
EDIT: Also check that the PlexMediaServer user has full control of the PlexMediaServer shared folder. This is taken care of when installing Plex. Don’t touch permissions for the PlexMediaServer folder.
https://forums.plex.tv/t/dsm-7-files-not-available/735455/17