It turns out that Google has changed their YouTube service yet again. The YouTube addon team spotted this a few days ago and created a fix - but if you attempt to use the addon before applying the fix then Google will flag your API’s as broken and block you from using it.
If you read this before using YouTube then download the linked ZIP file at this github location and install it and you should be good to go;
However if you used the addon before upgrading you will need to freshen your YouTube API after upgrading your addon to 6.8.14.
The procedure to freshen the API is quite simple.
Launch your YouTube addon and click on the sign in button.
This issues a code and tells you to login to google.com/device on the Android device linked to your YouTube addon (the one you created your API’s in) where you enter the code and re-authenticate the API. This is a HTML site so most browsers will complain about it been insecure, you need to go into advanced settings at the bottom of the warning and allow navigation to this site.
Thats it and things should be working on all Kodi devices within your local LAN network.
I also discovered today that it is possible to have API keys that allow for YouTube addon playback, search, live streams, however will fail to let one sign in to their account. Instead LoginException: [401] will show.
i am using coreelec 9.2.7 on an amlogic box with Kodi 18.9 - but the latest Youtube addon update i see in the official Kodi repository or anywhere else is version 6.8.11
Its not been pulled into the official repository yet so thats why a direct link to the developers github was supplied in the opening comment. Does no body read anything !!
in fairness i did read the first post - but that these updates were not pushed through to the main Kodi repository wasn’t stated - in fact other people seem to be able to get them from exactly that repository - hence the confusion.
anyway - looks like i have both the no-streams issue and the 401 wrong secret issue
The Youtube addon is available for both Leia and Matrix. Its clearly listed in the github. The Leia version will probably work on all older versions of Kodi, but thats for testing.