Which plex container are you running? Official, binhex, linuxserver, etc...?
You access to web client through https://app.plex.tv/ (remote plex web server) or local ip address (web client embedded in container)? Remote and local web clients are distinct and may differ.
Have you tried to start from a clean plex appdata folder? Stop the container, rename/move/backup plex appdata folder, start it again, claim your server (if needed), create a new library for the test and test a playback.
To rollback to your previous install, put back your old plex appdata folder.