There are a few different styles of web address (URL) created by YouTube depending on where you first saw the video, or if you clicked the share option. For whatever reason, the forums will almost always work properly when you use the actual web address of the YouTube video, and all of the other URL versions usually will not work.
Embedded - if the URL contains the word embed it means you are trying to link a full screen version of the video and will normally not work on the forums;
Featured - if the URL contains the word feature it means you clicked a link to the video after watching another video or from some other source where the video link was embedded and will normally not work on the forums;
Shortened - if the URL contains youtu.be it means you are trying to share the video using YouTubes share feature, which is logical but it is a shortened link and will normally not work on the forums;
HTTPS - if the URL starts with HTTPS:// it is a secure link and will not work on the forums, but if you delete the S so the URL starts HTTP://, the link will work just fine
You have to get to the web page that has a actual YouTube URL, this long-hand link will almost always link on the forums.
This worksActual link:
http://www.youtube.com/watch?v=iy-Xd7h4GAQThese do not workShortened link:
http://youtu.be/iy-Xd7h4GAQ Featured link:
http://www.youtube.com/watch?v=iy-Xd7h4GAQ&feature=youtu.beEmbedded link:
http://www.youtube.com/embed/iy-Xd7h4GAQ