looks like youtube embedding is simply broken, and i don't know at the moment what to do to fix it.
the plugin responsible for this feature hasn't been updated since 2011, yet the forum software has gone through a couple of update revisions since then. it's possible compatability between the two is getting shaky.
this forum software is going to be replaced with something new in a few months anyway. with the new forum software, youtube video embedding will be a natively enabled feature rather than require a plugin for it to work.
i'll try and figure out what the problem is, but i guess for now, consider this feature gone.