One last attempt to make this work before removing it forever SOLVED!

Plex Media Server Desktops & Laptops
6

  1. Knowing if the host is local to your LAN or remote from your LAN, is important because of how the client (Plex/web) works in searching.

  2. “localhost” is meaningless. Are you running the server on your computer? IF so, then ‘localhost’ has completely different context than if the server is running on another machine.

  3. As of Ubuntu 16, which includes Mint 18, /etc/default/plexmediaserver is no longer used. It is included solely to support Ubuntu 14 & 15 systems. The file for you to configure is /etc/systemd/system/plexmediaserver.service.d/override.conf

  4. You log from above shows the username it’s trying to use doesn’t exist. Did you edit /lib/systemd/system/plexmediaserver.service? If so, be advised, I will overwrite it with the next software update. This is why the override is required (also supported and systemd standard usage).

Oct 28 15:20:31 uber systemd[15511]: plexmediaserver.service: Failed at step USER spawning /bin/sh: No such process

If I may refer you to the full procedure for customizing your PMS installation which includes changing the username Plex runs as.

Which is often used in conjunction with: