I opened the current top five in the list of captures to confirm, they all do it on iOS for me. When I googled this, I wasn't finding anything. I have not tried on my android today btw.
It looks like this is still a thing. I get exact same error as OP's screenshot. Running iOS 10. Just to confirm, ALL youtube links on this forum show that error.
Edit to say: only reason I even care is because I'm in the habit of checking the cam captures section of the forum from my phone. Please help us - this is extremely important and crucial for a lot of super-important reasons.
Well not sure what version of the vbulletin script this forum is running , but at one of my forum is on vb 4.2.3 and youtube video runs fine on the mobile theme.