Plex/Web has been intermittently broken for me on the Wii U. I believe there's a pending support ticket at Plex to rectify the issue. Right now I really can't recommend using Plex/Web on Wii U. Any other device works fine (e.g. Apple TV, Chromecast, PlayStation 4, etc.) I've worked with Plex/Web team in the past to bring support to the Wii U, but the Wii U Browser is very limited so any changes to Plex/Web may severely affect functionality on the Wii U.