This is kind of a dupe of the question asked here: https://forums.plex.tv/discussion/191708/unable-to-cast-there-was-a-problem-playing-this-item
However, that question was marked as "Answered" without any solution actually being posted in that thread.
I have no lag or stuttering while casting, but randomly the video will just stop and the tv screen will say "Unable to cast - There was a problem playing this item". This problem has persisted since October, over several updates to both the Plex Media Server (Plex Pass versions) and the Android Plex app. In November I even build an entirely new computer and began using it as my Plex server. I have two v2 Chromecasts. Both exhibit this behavior. I never have an issue using Netflix or Youtube while casting. I also don't have an issue with just streaming a video to the Android Plex app on either my phone or my tablet. Sometimes I can get through an entire show before the error occurs, but it usually happens once or twice every show I watch. Sometimes it'll happen repeatedly, with 1 or 5 minutes passing between errors.
The other user posted his logs above, but didn't see anything out of the ordinary. I can post my logs as well if someone can point me to where I can find them. Obviously, the issue is pretty annoying and I've been dealing with it for quite some time. If anyone is able to help, that would be greatly appreciated.