Running Plex on Synology - some video's causing issues

I’m running Plex on my Synology 1520+ and a few of my movies are causing me some pain, they have different symptoms like:

  1. Sound but no video
  2. Neither sound nor video, just spinning wheel
  3. Video is reported as 14+ hours long (even though it is less than 2 hours long)

I’m playing the Plex movies located on my Synology, on my Apple TV, using the Plex App.
I’ve tried to locate if this could be related to a specific video format, but it does not appear so, at least I’ve not been successful at identifying this.