L5.6 MethodNotAllowedHttpException when user is logged out but sending a post request

I am using Sentinel for authentication. As long I am logged in, everything is working well. But when I am logged out (f.e. delete all values in the persistences table) and I am on something.blade.php and click a link that triggers a post request (please see the code snippet below), I will get forwarded to the login page. After login, I get following Laravel error: Symfony \ Component \ HttpKernel \ Exception \ MethodNotAllowedHttpException

Route in web.php:

Route::post('something/{xtype}/{xid}/execute/{yid}', 'MyController@execute');

Controller logic in MyController.php:

public function execute($xtype, $xid, $yid)
{
    // business logic
    return back();
}

View something.blade.php:

<form action="/something///execute/" method="POST" id="y_">
    {!! csrf_field() !!}
</form>

<a type="button" href="#" onClick="document.getElementById('y_').submit()">

Middleware AdminMiddleware.php:

public function handle($request, Closure $next)
{
    if (Sentinel::check())
    {
        if (Sentinel::getUser()->roles()->first()->slug == 'admin')
        {
            return $next($request);
        }
    }

    return redirect()->guest('/login')
    ->with(['error' => "You do not have the permission.."]);
    }
}

Edit:
After login, I will run into the LoginController and following code will be executed:

return redirect()->intended($fallbackUrl);

Since I am still new to Laravel, it is hard for me to debug deep inside the framework. Any ideas/suggestions from your side?
Everything is welcome! Thanks in advance!

via Chebli Mohamed