Plex: Thumbnails are loaded but videos do not play; spinner stays on screen indefinitely

Whether it is a Movie, Home Movie or TV Show, the thumbnails appear successfully, which means a connection to the Plex server running on my Mac is established. However, when I try to play any video, the spinner appears and keeps on spinning but a video never plays. This seems to be a common issue but so far I have been unable to find a solution. Samsung TV UN60F8000 Plex for Samsung 2.003 Mac OS X 10.10.4