Do you have it installed in a sub-directory /forums?
BTY, nice idea of your most recent install. I recently registered bogboys.com and will shortly being getting a license for it (already working on the style on my test server as well as the forum structure).
Compared my two forums again, and found the setting that fixed it for me. Set the index page route to forums/ instead of /forum. That removd the /forums at the end of the URL. Don't know why, but glad it is fixed.