Unfortunately our request to google to raise the daily limits, resulted in the API key to be invalidated. This is because we are not allowed to use youtube API to play audio without showing the video.
Bummer.
We are however already looking for an alternative solution.
In the meanwhile we are removing the youtbe plugin from the repository, to avoid confusion of people installing it without it working.
As soon as this is solved we will add it again.
Anybody which want to jump in and help is more than welcome, contatct us via the support page (select business proposal):
volumio.org/contact/
NOTE: The code method and personal API solution are not viable, since they do require advanced skill (we want everybody to be able to enjoy it)