You missed reading the DSM 7 release notes and migration instructions. Please read it so you understand all the changes.
Plex user is no longer used – DSM 7 forced the change. You must change permissions to grant “System Internal User” PlexMediaServer (See the FAQ below).
PlexMediaServer shared folder is for PMS use ONLY. IF you have placed media in the PlexMediaServer shared folder – MOVE IT OUT. We do not support (DSM limitation) placing media in PlexMediaServer.
If you are placing media in PlexMediaServer, MOVE IT out now please!
We do not support placing media there. There are warning files, written in 6 languages, at multiple levels.