You should never change permissions for the PlexMediaServer shared folder.
Please follow FAQ 32: Repairing the PlexMediaServer shared folder to restore permissions to their correct settings.
Do you still have the /Plex shared folder?
If yes, are the database file still present in /Plex/Library/Application Support/Plex Media Server/Plug-in Support/Databases?
If yes to both, then the migration from DSM 6 to DSM 7 did not complete.
See this post: Struggling with migration from DSM 6 to DSM7 - #2 by FordGuy61
