Trying to solve the YouTube embed problem for FE using WP 4.5.3.  Here are a couple of things you can try:

#1.  Use an iPhone or iOS device (maybe Android, don’t have access to one) to post your thread or reply.  Do NOT use any embed code or iframes, just paste the URL link straight from YouTube as text.  Wordpress will do the rest.

#2.  If a member posts a URL for a video and it does not convert, you as the Admin can go to Threads on your WordPress dashboard (under Pages and Comments on the left), and Edit the thread.  Go to the Text tab (not the Visual tab), then click the Update button, and the video will now appear.

I guess the Visual editor is causing the problem.

Hope that helps.  Maybe somebody can use these clues to figure out exactly what’s going on.