Hi, I wanted to make a post just in case it helps anyone else, if a video file refuses to play, try changing the file name to something simple. Here's how I found out... One of my seasons of Game of Thrones refused to play while the others had no issues. They were all the same file type too (MKV). The video would fail to start and Plex would throw a quick error on the screen and my android (to control the chromecast) would say something like "Media playback failed". So I dug through some logs and in my "Plex Media Server" .log file I found this line:
ERROR - Error opening file 'D:\Videos\TV\Game of Thrones\Game.Of.Thrones.S02.Season.2.1080p.5.1Ch.BluRay.ReEnc-DeeJayAhmed\Game.Of.Thrones.S02E01.1080p.5.1Ch.BluRay.ReEnc-DeeJayAhmed.mkv' - 2
So I renamed every file in the season to something simple like S02E01, S02E02, ... and it works now! I'm not sure if this is a bug in Plex, or a limitation of Windows or Chromecast. There might have been too many periods or hyphens in the file name or it was too long. Who knows. I hope this can help others.
On a side note, further in the logs it shows these errors right after playback which weren't too useful in debugging, just in case anyone saw these, check for the error about opening the file:
WARN - [Chromecast] [Player] Media element error code: 4
ERROR - [Chromecast] Load error: There was a problem playing this item.