Aparently, when defining a new route group I have to re-define the prefix. If I don't the prefix is no longer set.

So the problem is fixed by doing this:

/**
 *  Admin routes
 */
$app->group(['prefix' => 'admin', 'namespace' => 'App\Http\Controllers\WebsiteAdminControllers'], function($app){

	// The login page
	$app->get('login', ['as' => 'websiteAdminLogin', 'uses' => 'LoginController@getLoginForm']);


	// Private pages for logged in users
	$app->group(['prefix' => 'admin', 'middleware' => 'isWebsiteAdmin'], function($app){

		// Get the admin dashboard
		$app->get('/', 'DashboardController@getIndex');

	});

});