XMLRPC pingback.extensions.getPingbacks not work with parameters

Note that WordPress ships with WP_HTTP_IXR_CLIENT that extends the IXR_Client. So replace

require_once ABSPATH . WPINC . '/class-IXR.php';
$rpc = new IXR_Client( 'http://localhost/wordpress/wordpress/xmlrpc.php' );

with:

require_once ABSPATH . WPINC . '/class-IXR.php';
require_once ABSPATH . WPINC . '/class-wp-http-ixr-client.php';
$rpc = new WP_HTTP_IXR_CLIENT( 'http://localhost/wordpress/wordpress/xmlrpc.php' );

Then try replacing:

$result = $rpc->query( 
 'pingback.extensions.getPingbacks',
 'http://localhost/wordpress/wordpress/2018/01/17/hallo-welt/&post_type=something'
);

with

$result = $rpc->query( 
 'pingback.extensions.getPingbacks',
 'http://localhost/wordpress/wordpress/2018/01/17/hallo-welt/?post_type=something'
);

The server method wp_xmlrpc_server::pingback_extensions_getPingbacks() uses the url_to_postid() function to convert the url to a post ID. So make sure your url works there!

PS: These days many WordPress users are looking into the new REST-API instead of the XML-RPC API.