He simply used a fresh install of Operating System and Plex server.
But I don’t think that this “nuclear option” is the right thing to do.
Start by using the hosted web app app.plex.tv instead of either the local web app or the Plex for Windows/Mac app or PlexMediaPlayer.
Disable any web browser add-ins which could potentially interfere
Clear the agent caches app.plex.tv https://support.plex.tv/articles/202967376-clearing-plugin-channel-agent-http-caches/
Set your router to use an alternative DNS server. (Granted, a long shot. But could still be helpful.)
Check for other filter mechanisms in your router, your anti virus or even your ISP’s preferences.
