[Bagi Zoltan]

I faced with the very same problem with two bbpress installations along with a WP. I slightly modified the function is_404() in the /wp-includes/query.php so the function returns true when the requested uri belongs to the forums.

function is_404 () {

global $wp_query;

if (preg_match('/(firstbbpressfoldername|secondbbpressfoldername)/i', $_SERVER))

{return true;}

else{

return $wp_query->is_404;}

}

After saving the irritating 404 issue has gone.

Regards

Zoli