I would try to completely uninstall Plex for Mac, reboot, then install it again.
Because I do that a lot for testing, I have a script I call delplex.sh
#!/bin/bash
echo ' '
rm -rf /Applications/Plex.app && echo '[1/5] deleted the app'
rm -f ~/Library/Preferences/tv.plex.desktop.plist && echo '[2/5] deleted the plist'
rm -rf ~/Library/Logs/Plex && echo '[3/5] deleted the logs'
rm -rf ~/Library/Caches/Plex && rm -rf ~/Library/Caches/tv.plex.desktop && echo '[4/5] deleted the caches'
rm -rf ~/Library/Application\ Support/Plex && echo '[5/5] deleted the application support'
echo 'Plex for Mac has been removed.'
echo ' '
