This has been happening for months so even though everything below is the latest versions (both plex and OS)
21:26:15 launch PMS 1.3.3.3148 on Mac Mini running OSX 10.12.1
21:26:46 launch PMP 1.2.1 on MacBookPro running 10.12.1 (though happens on 2nd Mac Mini)
21:27:34 launch iOS 4.6 on iPhone 6 running 10.2
21:28:06 iOS: selected PMP as player
21:28:46 iOS: select movie (also showed on PMP)
21:29:05 last log event in PMS logs
21:29:20 iOS: played movie, immediate error: "Couldn't retrieve the play queue for this item. Please try again" (on ios)
21:30:00 iOS: went back to home
21:30:24 iOS: select movie, play movie PMP error: "Media not found". PMS is crashed (may have been since 21:29:05)
21:30:27 Crash Uploader reported 7440149E-6012-4A2C-A55F-0415B32BCBFE