pingback returns faultCode 0, no message

For some reason, the default filter attached to pingback errors, will not send an error message unless the error code is 48. From wp-includes/commment.php:

function xmlrpc_pingback_error( $ixr_error ) {
    if ( $ixr_error->code === 48 )
        return $ixr_error;
    return new IXR_Error( 0, '' );
}

The standard Wordpress source only contains a single pingback error call with a faultCode of 0 and an empty message, in wp-includes/class-wp-xmlrpc-server.php:

} elseif ( is_string($urltest['fragment']) ) {
        // ...or a string #title, a little more complicated
        $title = preg_replace('/[^a-z0-9]/i', '.', $urltest['fragment']);
        $sql = $wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE post_title RLIKE %s", like_escape( $title ) );
        if (! ($post_ID = $wpdb->get_var($sql)) ) {
                // returning unknown error '0' is better than die()ing
                return $this->pingback_error( 0, '' );
        }
        $way = 'from the fragment (title)';
}

So the error can happen when the post cannot be determined from the URL and there is a fragment/anchor (#foo) at the end of the URL. That fragment is interpreted as title and the posts table is searched for a post with exact this title.

Unfortunately, this can't be the reason in my case since I don't have anchors in the URLs.