This can happen when the first-run is accidentally rushed after the ugprade
(It has a LOT to do underneath what you see in package center)
The easy way to reclaim / get control again is now built into the DSM 7 package and it’s menus 
Please don’t play with the PlexMediaServer shared folder.
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The username isn’t “Plex” anymore. It’s “PlexMediaServer” on DSM 7.
– If not careful, you’ll break it.
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You will need to grant permission for PlexMediaServer to read your media (This is a Synology-forced change)