None of these solutions worked for me. It seems to be the way that Siri is sending search results to open Youtube.
It works if I tell Siri "Look on youtube for..." and it displays a grid of videos to click.
If I just click the Siri search result after saying "hey siri...i have this question..." then click whatever link there for a youtube, then it doesn't work, every time. I have tried all the suggestions to fix it. It is something in the way that the search is being sent from the Siri result, through google, to the youtube page.
I'd love to see more complaints about this, it does seem to be an apple issue in the way searches are being redirected from the Siri result to open a youtube search result from Siri.
It has nothing to do with location or device, it happens on multiple friends' ipads and iPhones I've tried, and happens whether or not I'm using Wifi or cellular and happens from multiple locations on cellular and wifi.