Interesting… the BuddyPress forums (https://bbpress.org/forums/) have the same problem, but the bbPress.org forums are fine.
I know that the BP forums and our site both use deep integration, so I tried removing that. Result: No more 404.
So it appears that deep integration is part of this problem.
I have bbPress 1.0.2, WPMU 2.8.4a and BuddyPress 1.0.3.
Looks like WordPress is setting the 404 because it doesn’t recognize the bbPress page that is loading.
When the global WP object is constructed, the function WP->handle_404 calls “status_header( 404 )”.
As a workaround, I modified “handle_404” to lay off the 404 if “BB_PATH” is defined (i.e. when a bbPress page is loading). This seems to work ok.
Here’s the new “handle_404” at around line 445 in “wp-includes/classes.php”.
function handle_404() {
global $wp_query;
if ( !defined( 'BB_PATH' ) && (0 == count($wp_query->posts)) && !is_404() && !is_search() && ( $this->did_permalink || (!empty($_SERVER['QUERY_STRING']) && (false === strpos($_SERVER['REQUEST_URI'], '?'))) ) ) {
// Don't 404 for these queries if they matched an object.
if ( ( is_tag() || is_category() || is_author() ) && $wp_query->get_queried_object() ) {
if ( !is_404() )
status_header( 200 );
return;
}
$wp_query->set_404();
status_header( 404 );
nocache_headers();
} elseif ( !is_404() ) {
status_header( 200 );
}
}
