Here's one thing to watch out for. Say you open a web page, somewhere, and on it is a youtube video and you start watching it, then decide to open it on the youtube website. Don't use that URL. It will have a time_continue parameter that is great for continuing where you left off on the page you found it but it will cause the forum to not display anything.
Time_continue looks like this: https://www.youtube.com/watch?time_continue=29&v=Yq5ttYcb72s. And will show nothing:
https://www.youtube.com/watch?time_c...&v=Yq5ttYcb72s
What you need to do is remove it and the trailing & (keep the ? and v etc) so it looks like this: https://www.youtube.com/watch?v=Yq5ttYcb72s. And then it should show:
That won't fix the issue being discussed here, just eliminate one cause.