I had this too, only thing that worked was clearing cookies and site data of YouTube, then logging in again. Another browser on the same account with the same uBlock settings worked the whole time, so I guess it's "device" bound not account bound. Since the cookie purge I don't even get the AdBlock warnings anymore.

I’m able to play videos on my Samsung S9 with Firefox running with ublock. I never got a message.

I'm unable to watch any video despite uBlock instructions thing.

For some reason, I got this popup but I didnt care which then blocked my player but I tried several different adblockers simultaneously which made the ads not play but there was a black screen for 5 seconds or so when the ad should've played but then I installed adguard, which after that no matter what I did on youtube, didn't get any ad anywhere or the popup, with the ad blockers on or off .

Using Vivialdi btw