Just updated my MAC OS to Sierra (10.12) today. Now when trying to play some of my movies (bluray rips), I get the message "There was a problem playing this item". I thought it might some sort of size restriction, but I tried playing various movies of different sizes and there was no consistency as to why some play and some don't.
I'm running Plex Server on my MAC, Version 1.1.4.2757 , and again, no issues until after the Sierra updated completed today. Interesting thing is, I can play them on all Roku devices and iPhone, but not directly on the server which is what I use when I'm in my office working. I can open the file directly in VLC, no issues at all.
Also, I am running a windows version in my office as well for DVDS. When I try and connect to my BLURAY server and play movies from that server, no issues, works fine.
So this seems to be isolated to my MAC server only for local playback.
I'm sure the logs would help, so please let me know what log you need and I will be happy to post.