Laravel 5.1 Auth not consistent

I am working on a project and did the Laravel 5.1 Authentication. I followed the Documentation. I only have one user, no registration.

Now, when I protect routes I do it this way:

Route::group(array('middleware' => 'auth', 'prefix' => '/admin'), function() 
{ ... }

Then the Login-Form comes back, i authenticate and it is working. But now, when I refresh the page (or go to other admin-pages) sometimes I am logged out and redirected to the login-form again, sometimes I get an "Whoops, looks like something went wrong.", and sometimes it is working for some requests.

I have no idea why and when it is happening. I followed this documentation: http://ift.tt/1NTLsZh

Did someone have the same problems? When I was working with Laravel 4.* everything was fine :( Thank you already!! gerti

via Chebli Mohamed