... Nothing of this sort here... Trying a non-protected, non-music video like
https://www.youtube.com/watch?v=I6M_vYUUx20
I can fetch without issues:
I was curious to try a "protected" music video from VEVO, like
https://www.youtube.com/watch?v=BilaShsQphM
Even HD versions are available to download, provided I first select them in YT's embedded player (quality settings)
Those MKV files contain VP9/opus encodes; if you are instead after a 720p MP4 file encoded in h264/aac, then you first have to disable (temporarily) Media Source Extensions (MSE) in NM28's settings
Options -> Content tab -> Video -> uncheck MSE
so that YT serves you that type of stream:
If you're that interested in YT content, I'd recommend a specialised tool (not a browser plugin) like the constantly updated CLI app youtube-dl, if you're willing, that is, to work in the Windows Command Prompt...
You should've created a separate support post and let @roytam1 know about this issue; needless to say, you should PM him log-in credentials too, if any hope of debugging this... Moonchilld have EOL'd Tycho (PM27), so 0 chances they address this now...