[curael]

Adding API keys is not working for me. All I get is "LOGINEXCEPTION:[401] UNATHORIZED_CLIENT". These are new keys (I didn't have a use for them before). I've turned off "use developer keys". I've used copy and paste to make triple sure that I copied the correct credentials in for API key, AIP ID and API Secret. I've tried uninstalling (though Kodi doesn't allow me to uninstall the YouTube addon due to a dependency). I've tried clearing and deleting all the settings etc and running the setup wizard.

It's just not working for me.

Here's to hoping an update can fix this problem for all of us.

Did you do "remove user" after you added the API key? I had to reregister the app from scratch after the keys were inserted. And then it worked. You could also try deleting the addon from your OS instead of inside Kodi. Since you can't remove it. But for me, I didn't have to remove anything. Just remove the "default" user in the addon settings and then do the two auth requests again on google.com/device or what ever. The name of the API access should then show up during registration.     

There is one part I am unsure of...somewhere in the oath setup about, you pick devices and access. I had to click to add zone access (otherwise its just email and profile). I do not know if that is required to run the app, but I enabled all the locked youtube. That means they will review it at some point, but you might not need to do that to get it to work.
when you reboot, you should get the 2 google auth screens.

    

Yes, if you pick a "zone" you are applying restrictions, and that starts the verification process on the key. You do not need to add any zone for this to work. So it's not really a good idea to enable it in this case. It only complicates your process.

Tinfoil hat here. So with the solutions posted on the last few pages, did Google enforce a change to their API that makes it impossible to access it without providing identifying credentials like a key you created in a project? Do they really hate guest users they cannot identify that much? Is it still somehow possible to access YouTube with this addon without providing something that is tied to a project which can be used to connect to you?

Is public exchange/posting of keys an option?

There seems to be only one option for you, and that is to create the API key for YouTube API v3 with only public data access and not personal user data. This way you put that key in and YouTube will start working without having to sign in. How ever, this will still require a Google account for creating the API access. So this is definitely not a good solution for public places or the likes. But yes, that way you can actually have the add-on work, wihtout it showing your subscriptions or anything and "Search" and "Popular right now" will work and you will have no need for signing in - or I should say, as opposed to the User data, you simply can't sign in - so the API key isn't as useful if someone were to get it. It doesn't grant access to your user data. The solution for the Google account could be creating an account specific for this and then just don't touch it after you generate the key. It's a hassle and I agree that it's rediculous of Google to do this. But oh well, what can ya do. Security is security and they are doubling down on it, it seems.