Error: Data structure returned is not convertible in the requested format: Multidimensional column values not supported. Found unexpected array value for column 'category' in row '0': 'array ( 'id' => 'General_Actions', 'name' => 'Behaviour', 'order' => 10, 'icon' => 'icon-reporting-actions', 'help' => '', 'widget' => NULL, )'. Try to call this method with the parameters '&format=original&serialize=1'; you will get the original php data structure serialized.