This is "Ad blockers violate YouTube's Terms of Service" message. I'm getting it across multiple devices where I use Opera (which very effectively blocks all youtube ads).
This is a massive problem, because even if I disable all ad blocking, it still doesn't leave me alone if I open YT in Opera. In fact, it looks like Google are linking it to the user somehow. If I log out, I'm not getting this. But as soon as I log in, boom, all videos are blocked.
Has anyone found a way around this?