Latest Android 5.0 updates and Serenity
If you use Serenity on an Android TV or Android 5.0 supported device, and use MX Player, MX Player is currently broken on these devices so until this is fixed, the app can only play back MKV/MP4 with H.264 video and AAC audio tracks. Hopefully as other Android TV devices come to market, more codecs and containers will be supported natively. Until then you can try other external players to play the content, or just play the supported content with the internal player. Other players though will not provide the necessary information in regards to playback position that is needed to update the Plex meta data for the video.