After the last most recent auto-update of Emby for Media Center, I started having a problem with just the Mp4 movies, but not the TV episodes. I press the Play button on the menu and get a dark screen. When I move my mouse I see the control buttons on the bottom of the screen (Stop, Play, Record, etc) but no time length indicator. The Play button displays the || pause like the movie is playing. I press the square Stop button which takes me back to the movie info page and buttons, then I get the spinning wheel and the page goes dark and the time indicator appears and the movie starts up. This is an annoyance, but at least I can get them to play.
I have uninstalled/reinstalled E-WMC and the server both. I am still getting that small hang on the movies so far. TV episodes run right away. These are all the same MP4's I have for over a year or more.