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.