The Plex Staff are not commenting on the Xbox 360 as the app is no longer allowed to receive updates per MS and is no longer receiving development (as of Oct 2017). See here: Plex for Xbox 360 Status Update
The previous comment stated going back to 1.18.0.1944 for PMS seemed to fix the issue. Obviously not the recommended way to fix it, but will likely be your only fix if you insist on remaining on the 360.
