Error:  Data structure returned is not convertible in the requested format. Try to call this method with the parameters '&format=original&serialize=1'; you will get the original php data structure serialized. The data structure looks like this: 
 $data = array (
  0 => 
  array (
    'login' => 'anonymous',
    'alias' => 'anonymous',
    'role' => 'view',
    'capabilities' => 
    array (
    ),
  ),
);