I'm surprised no-one from the tech team has replied to this yet. I'm having the same problem and I was about to post a new thread when I saw that you'd already raised one.
Is it anything to do with Youtube's change from using http:// to using https:// ?
I'm using V2.1.5 Rincewind and the debug (verbosity 2) log says:
I've edited all occurrences of the URL except the first to %URL% because of the forum's error message 'Your message contains too many URLs. The maximum number of URLs allowed is 5.'
main debug: adding item `
https://www.youtube.com/watch?v=O2r2HtftC6w' (
https://www.youtube.com/watch?v=O2r2HtftC6w )
qt4 debug: Adding a new MRL to recent ones:
https://www.youtube.com/watch?v=O2r2HtftC6w
main debug: meta ok for (null), need to fetch art
main debug: processing request item: %URL%, node: Playlist, skip: 0
main debug: looking for meta fetcher module matching "any": 1 candidates
main debug: rebuilding array of current - root Playlist
main debug: rebuild done - 4 items, index 3
main debug: starting playback of the new playlist item
main debug: resyncing on %URL%
main debug: %URL% is at 3
main debug: creating new input thread
main debug: Creating an input for '%URL%'
lua debug: Trying Lua scripts in C:\Users\Martin\AppData\Roaming\vlc\lua\meta\fetcher
main debug: using timeshift granularity of 50 MiB, in path 'C:\Users\Martin\AppData\Local\Temp'
main debug: `%URL%' gives access `https' demux `' path `
www.youtube.com/watch?v=O2r2HtftC6w'
lua debug: Trying Lua scripts in C:\Program Files\VideoLAN\VLC\lua\meta\fetcher
main debug: creating demux: access='https' demux='' location='
www.youtube.com/watch?v=O2r2HtftC6w' file='\\www.youtube.com\watch?v=O2r2HtftC6w'
main debug: looking for access_demux module matching "https": 12 candidates
main debug: no access_demux modules matched
main debug: creating access 'https' location='
www.youtube.com/watch?v=O2r2HtftC6w', path='\\www.youtube.com\watch?v=O2r2HtftC6w'
main debug: looking for access module matching "https": 20 candidates
main debug: looking for tls client module matching "any": 0 candidates
main debug: no tls client modules
main error: TLS client plugin not available
main debug: no access modules matched
main error: open of `%URL%' failed
main debug: finished input
main debug: dead input
lua debug: Trying Lua playlist script C:\Program Files\VideoLAN\VLC\lua\meta\fetcher\tvrage.luac
main debug: changing item without a request (current 3/4)
main debug: nothing to play
main debug: using meta fetcher module "lua"
main debug: removing module "lua"
main debug: searching art for %URL%
main debug: looking for art finder module matching "any": 2 candidates
lua debug: Trying Lua scripts in C:\Users\Martin\AppData\Roaming\vlc\lua\meta\art
lua debug: Trying Lua scripts in C:\Program Files\VideoLAN\VLC\lua\meta\art
lua debug: Trying Lua playlist script C:\Program Files\VideoLAN\VLC\lua\meta\art\00_musicbrainz.luac
lua debug: Trying Lua playlist script C:\Program Files\VideoLAN\VLC\lua\meta\art\01_googleimage.luac
lua debug: Trying Lua playlist script C:\Program Files\VideoLAN\VLC\lua\meta\art\02_frenchtv.luac
lua debug: Trying Lua playlist script C:\Program Files\VideoLAN\VLC\lua\meta\art\03_lastfm.luac
main debug: no art finder modules matched
main debug: art not found for %URL%
main error: TLS client plugin not available
main error: open of `%URL%' failed