i installed Plex on my Synology DS214se, DSM 6.1-15047 Update 1. My Plex instance is 1.4.3.3433-03e4cfa35. The install went fine but when trying to watch on my Samsung TV using the Plex app in Samsung app store, the movies wont fully load. I see the squares where they should be but images don't show up and when I try to play one, it just spins and spins and never plays. I went through the FAQ post here and confirmed I have settings correct but it was already set as you described.
Ideas?