I had this error displaying to me when attempting to play a few items from my library. The files I knew were OK as VLC playback via sshfs direct from the server was fine.
Read a few things about SSL connections, Subtitles, this, that and the other – but none were the cause of my issue.
Turns out it was disk space.
From ‘Plex Media Server.log’
Feb 09, 2016 10:55:41 [0x7fc28a3f9700] WARN – Low disk space: 5043020343 bytes source file, 18746441728 bytes capacity, 3664343040 bytes available on /var/lib/plexmediaserver/Library/Application Support/Plex Media Server/Cache/Transcode/Sessions
Feb 09, 2016 10:55:41 [0x7fc28a3f9700] WARN – TranscodeSession: inadequate disk space for transcode
Feb 09, 2016 10:55:41 [0x7fc28a3f9700] ERROR – Failed to start session successfully.
Who’d have thought only having 3.5GB~ free would be an issue!
Just thought I’d share.