This is rather strange. I have just had a user report exactly the same thing to me (with a screenshot to prove it). I've tried with the URLs that he had issues with:
[CODE]https://youtu.be/QF031DwMffQ
https://www.youtube.com/watch?v=QF031DwMffQ&feature=youtu.be
https://youtu.be/6340tX9M_eM
https://www.youtube.com/watch?v=1c0Th8-809k[/CODE]
The user says that he has issues on his work computer (which is Windows XP and has old versions of Firefox and Chrome that run on that) but also the same problem on his phone and tablet which are running the latest version of Chrome for Android. He is using various ISPs - BT, Virgin and Vodaphone. He said he noticed the issue first late last week (which rules out the XenForo 1.5.16a upgrade I did yesterday) so it seems to be related to XFMG 1.1.15/XenForo 1.5.16.
As a further test, the user put the URLs into a forum post and there was no problem there.
There are no required custom fields used at all in XFMG on my site.
I have tested all of this on my site with these URLs and I cannot replicate the issue at all. I've chatted with the user using our chat facility and got him to try various things and he's still got these issues, yet I'm having no problems with the same URLs (I'm on Virgin and EE).
