When trying to watch a movie streamed by the Plex Server of my DS713+, I used to get an error "Cannot load M3U8: 404 not found" (only on mobile devices or within a browser). This was due to a wrong "temp" directory configured (out-of-the-box) for transcoding purpose.
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Watching movies was working fine with the Plex Theater on my PC.
I had the error message "Cannot load M3U8: 404 not found" only when trying to play movies within Chrome (on a PC or on a tablet).
I read on the web that this error could be due to the NAS struggling with transcoding the movie (which is done or not depending on the client's spec). Plex clearly states that one needs a dual-core NAS to be able to do transcoding. But I have a DS713+. So it should have been fine
I also read that Plex Theater (on Windows) does not need any transcoding. As it was working fine, I definitively though there should be something wrong with the configuration of the transcoding...
I did look at the logs of Plex Server (in \\<MyNas>\plex\Library\Application Support\Plex Media Server\Logs) and found that error:
Checking directly on the NAS via telnet, I found that the right path should have been /volume1/Plex/Library/tmp_transcoding (Notice: "Plex" starts here with a upper case although the related shared folder accessible via SMB starts with lower case!!!)
So, I did force that correct value via the Plex server web interface > Settings > Transcoder > Show Advanced Settings > Transcoder temporary directory.
And BINGO !!!