Just got my Plex server set up on my RasPi 3B+, Stretch. Attached my external hard drive and Plex built the library. However, when I go to play any media, I get errors and failure to load. My FireTV Stick app says "Unable to play media". My windows laptop, both through the app and the Web interfaces, say "Playback Error, An error occurred loading items to play".
The Pi is on wired ethernet and the network connection is strong.