Plex media player doesn't play or even recognize AVI files. If they are recognized (maybe due to file renaming) they won't get played on several players as they need to be converted on the fly.
If Plex for instance is running on AV low-end hardware - in my case a Synology Diskstation - there's no power to do this conversion on the fly.
Convert files in advance using
libav-tools on Linux.
avconv -i test.avi -c:v libx264 -c:a copy outputfile.mp4