When I create a topic, I can post a YouTube video with no issues.  But when I attempt to add a "Reply" to the same topic I get the "403 Forbidden" error.  I have tried turning mod_security off, but to no avail.  Any suggestions?

You will need to contact your hosting provider as a 403 Forbidden error is likely from a security module, access to YouTube, or otherwise. They will be able to further assist you.