…I’m pretty sure YouTube’s safe. (I hope that I’m not actually the fool in this case and that this isn’t a symptom of some larger underlying problem with my browser being infected or something, but I’m fairly certain I’m clean.)
a link in youtube comments pointing to a blocked URL;
infected browser (for example extension that we block);
MitM attack (router or ISP) injecting blocked URLs to your content.
Does it still happen? Does it happen in a different browser? On a different device in the same network? Does it happen on the same device in a different network?
I have been able to find it in stats database. It is caused by this blocked URL: connermcd[.]com (the website of the author of the video). I am unblocking it now!