Bug wp_insert_category / get_categories

wp_insert_category is not working or reading from a freshly installed wordpress:

require("wp-load.php");
require("wp-blog-header.php");
require("wp-admin/includes/taxonomy.php");

inserting top-level (parent=>0) categories works and the list is being displayed.
if I add a category to an already existing parent, it will not show up in the category view (but in the parent dropdown while creating a new category it does, aswell as in category list while writing a new post/page).
after I create a new category via admin menu, all categories show up normally.

please fix

PS: the categories are present in the database