Plex dead after upgrade to DSM 7.0 - 32400 port error

You might have missed the change documentation , as well as that final popup when the package installs.

It tells you :

  1. If Plex can’t see your media
  2. Give System Internal User PlexMediaServer permission. (this is the biggest change in DSM 7. We were forced to change the username)

The complete documentation thread is here. It has all the changes and procedures.