Routing error in CI 4.0.0-rc.1

Hello everybody,
first, don't be affraid of my English 
I've read lot of topics for my problem but i've found no solution 

I've install CI 4.0.0-rc.1 on my server, configure for development environment and try to acces to my project : no problem i have the CI welcome page ... good !!!

After that, i've create my first "very simple" controller in app/Controller named Admin here's the simply code !!!

<code>
<?php namespace App\Controllers;

    use CodeIgniter\Controller;

    class Admin extends Controller
    {
        public function index()
        {
             echo 'Admin area';
        }
    }

</code>

I put this line in app/Config/Routes.php :

$routes->add('admin', 'Admin::index');

and when i go to myproject/admin i've got this message on a white page (not the pretty CI 404 page) : "File not found" (you can see this at https://dev.cdsi-formation.fr)

I've testing $routes->add('admin', 'admin::index'); (all in lowercase) but it doesn't work to ...

I don't understand why that doesn't work ... please hellllllp 

Thanks a lot for your future help 

[edit]
i've test that in local and ... it works fine !
In local i've run PHP 7.2.21 and 7.2.22 on the sever
Is there a special server configuration for CI4 working ???