Display validation errors in Laravel 5.2 - $errors variable is empty

I am trying to display validation errors when going back to a View, in Laravel 5.2. The $errors variable is empty and in a similar question Laravel 5.2 $errors not appearing in Blade it has been proposed to:

(1) move the ShareErrorsFromSession middleware to the $middleware property

or

(2) wrap the relevant route with web middleware.

Of course, solution 1 (which works) is not applicable in my case because I have some API routes, as it is well stated in Laravel 5.2 validation errors.

To be more specific, I am using multiple authentication guard drivers and routes that use token authentication are stateless. So what us left is solution 2. It should work, but it doesn't work (OR I am missing something). No matter if I place my routes (the ones in which I want to display validation errors) inside the group:

Route::group(['middleware' => ['web','auth:web']], function () {        
    ...
} 

or just inside:

Route::group(['middleware' => 'web'], function () {
    ...
}

the $errors variable seems empty. Dumping the variable gives:

object(Illuminate\Support\ViewErrorBag)#209 (1) {
  ["bags":protected]=>
  array(0) {
  }
}

The web group is typically defined in kernel.php as:

protected $middlewareGroups = [
        'web' => [
            \App\Http\Middleware\EncryptCookies::class,
            \Illuminate\Cookie\Middleware\AddQueuedCookiesToResponse::class,
            \Illuminate\Session\Middleware\StartSession::class,            
            \Illuminate\View\Middleware\ShareErrorsFromSession::class,
            \App\Http\Middleware\VerifyCsrfToken::class,
        ],

        'api' => [
            'throttle:60,1',
        ],
    ];

My controller looks like:

$form = Input::all();   
$rules = Config::get('validation.contact');            
$validation = Validator::make($form,$rules);

if ($validation->fails()){             
    return Redirect::back()->withErrors($validation);
} else {
    ...
}

Any ideas?

via Chebli Mohamed