I have had that issue for a couple of weeks on my ipad (but not on my iphone).
Prior to that (dunno if linked but), for several weeks, I had regularly the usual "adblock detected" screen when watching a video, but clearing cookies and cache would make YT leave me alone for a few days.
also, when this specific issue first showed up, refreshing the video or even opening YT in private mode would still show the error.
I tried today, and refreshing does launch the video as usual, and private mode doesn't show this screen at all (but here, clearing cache & cookies doesn't do anything)
And as previously said, I have that issue only on my ipad (1.4.4 (2) webkit 8621.4.1.10.1), not my iphone (1.4.4 (2) webkit 8613.7.1.0.13).