When you play a TED Talk, the Plex app is directly passing the URL of the content to the Media Foundation playback engine. We basically get out of the way. So if this is failing, it's almost definitely because (a) Media Foundation is having serious issues of its own, or ( there's a network issue preventing you from connecting to the TED video.
This is why I was asking you whether other modern apps seem to be able to connect to online sites - e.g. there's a YT app, isn't there?
You should confirm that you're DirectPlaying by swiping/right-clicking the appbar while the dots are progressing. You'll get some minimal information about what the app is doing (e.g. DirectPlay vs transcode, the state of the Microsoft Media Foundation playback engine).