title: Magento doesn't show all the categories in admin
body: <p>we have a magento store with various categories one inside another (subcategory). Our problem is that when we enter in the admin to manage categories in the category tree on the left, some of our categories that has subcategories looks correctly with the plus (+) icon on the left but when we try to expand the category magento doesn't display any item.</p>
  
  <p>The ajax call point to this url:</p>
  
  <blockquote>
    <p>index.php/admin/catalog_category/categoriesJson/key/09b218741dce69171825fdbf4954855d/?isAjax=true</p>
  </blockquote>
  
  <p>and it returns an empty array without throwing any error. Frontend displays all the categories correctly.</p>
  
  <p>Magento version 1.4.2.1</p>
  
  <p>Any idea ?</p>