Hi,
I just started using Yatse. Adding Plex worked fine, but I can’t get it to play om my phone. When I press the “cast” button, I only see my Sonos speakers…
What am I doing wrong?
You have nothing to select, when you are connected to Plex the default player is local device since Plex is not a player.
OK, then that’s not the problem 
When I press play on an episode nothing starts though. It looks like it’s trying to play on an external device…
It start on a video player.
As the template said please provide logs. And be sure to have a player like VLC or Mx Player.