This is affecting me too. plexmediaserver 1.26.0.5715-8cf78dab3 on Ubuntu 20.04.4 LTS.
Tuner is HDHomeRun EXTEND (HDTC-2US)
Cast device is a Gen2 Chromecast, running System firmware 275994, Cast firmware 1.56.275994.
Casting local content works (including shows recorded from the same tuner) but Live TV doesn’t and gives an error “Sorry! Something went wrong”
Tried various “Transcoder Quality” in Live TV & DVR Device Settings, same problem
“Original format”, “Highest quality”, and “Low quality (240p)”
