Following a Google hit I arrived at a archived topic, which worked fine. However when I choose the forums link(i.e Home >> Forums >> Archived: Hardware) it 404’d. Selecting Home worked just fine.
It looks like someone just needs to alias /forum-archive/forum.html to /forum-archive/index.html, or even simpler, sym-link forum.html to index.html.
Ok, Home(aka /forum-archive/index.html) does not contain the “Forums” link. But as far as I can tell, every page below does contain the invalid link.
And while the way your depth is displayed is a little confusing, if /forum-archive/forum.html was linked/forwarded to /forum-archive/index.html at least you would not be black holed/dead ended by the 404.
Sorry for the bother, but I just wanted to tell someone about the dead link(s).