@Adblock Systems:
I'm using this addon for years because it's doing its job very reliably.

Alas, YouTube seems to have changed their in-video ad mechanism (again *sighs*) recently (as in early to mid-may 2021), because in-video ads start to creep back into YT videos:
Since about a month, I experience short blackouts in videos when YT tries to show me an ad,
since about one week videos are interrupted and I see ad for about a second (or still pictures of preroll ads before I start a video),
and since about two hours (late night of june 2nd CET), ads sadly aren't blocked anymore at all.

So I guess this addon needs another update to work properly - again *sighs*. Fudge you, YouTube.
Well, won't be the first time, and won't be the last, I suppose.

Nevertheless, I'll keep the addon and still do recommend it. Because it might not work right now, but in the past years, I experienced that such issues were always resolved in a matter of days or worst case weeks).
And that is why I also still rate this addon with five stars: because this addon already earned them anyways, and judging from my experience with the addon for many years, such issues were always fixed in a reasonable time, so I'm quite confident that it'll work again soon.

So, I'm looking forward to hopefully being able again soon to forget again that YouTube fucks up videos by interrupting them with pesky ads I'm not even remotely interested in. ;-)

BTW: Is posting URLs of videos where ads weren't blocked any help in updating the addon?
Because I just had an in-video ad from IKEA at about 3:00 in this video: https://www.youtube.com/watch?v=vOrnsrUJg58
(If that doesn't help, please just say so, and if it does, please do so too.)