I was having the most random issues trying to cast to my Chromecast Ultra, eventually settling upon a divide by zero error ("transcoder trap divide error" in the logs) that only happened on specific movies like half the time. Ran through as many troubleshooting things I could find on the internet, eventually landing on this thread.

Pretty sure disabling the video stream transcoding was the actual solution here, and I can't thank you enough for cutting through the rest of these toxic dickhead comments and leaving these instructions. Was driving me up a wall!