Make sure that both starting URL and your forums URL reside on *exactly* the same domain, including www. prefix if required, i.e. domain.com and www. domain.com are *different* domains.
I just tested this and have become even more confused. I have always used [external links are visible to admins only] for my index page and [external links are visible to admins only] is where my forum resides. I'm assuming that this complies with your example and is correct. Just for fun I typed in [external links are visible to admins only] , and my indexpage showed up. Baffled I typed in [external links are visible to admins only] and my forum popped up. so how is this explained? Is it because of my browser history and cache, cookies or what? At any rate I'm no closer to solving anything.
Thanks