Ajax get request works, but post fails, same url.

You have forced it into get(), and you are making a post().

https://codeigniter4.github.io/userguide...-in-routes

PHP Code:

$routes->group('navigation', ['namespace' => 'Modules\Navigation\Controllers'], function($routes)
{
   $routes->post('/', 'NavigationController::index');
   $routes->post('getnavigationitems', 'NavigationController::getNavigationItems');
});