Re: There are no 404’s
My guess is that there is something in the WordPress .htaccess file that is causing it to mess with what should be a forum 404 error. I don’t know enough about subdomains and bbPress integrated with WordPress, but that’s my guess. Something in the WordPress .htaccess file is rewriting your bbPress 404. Possibly? Is there a .htaccess file for bbPress? I would guess not, since you’re not using permalinks for the forum.
I suspect it has something to do with the subdomain, but that’s just a SWAG.