I'm having a similar problem with my themes. You can see the problem in Wordpress 2.8 if you try (in the functions.php or a theme file)

print_r(get_categories('hide_empty=0'));

I use an array of categories for a select input field in my theme options page. Since WordPress 2.8 it does not work any more.

The functions returns:

Array
(
    [errors] => Array
        (
            [invalid_taxonomy] => Array
                (
                    [0] => Invalid Taxonomy
                )
        )
    [error_data] => Array
        (
        )
)

Another discussion on this:
http://themeforest.net/forums/thread/wp-28-compatible/13432

Anybody has an idea how to solve this issue? Thanks in advance!

Simon

I used to have

$index_cats = '16,7,5,10'; // What categories to show on Index page
$args = array(
'orderby' => 'RAND', //ID
'order' => 'DESC', // change cat ids so to get right order
'hierarchical' => 0,
'include' => $index_cats ); //Which categories to include, set on the theme options page

$cats = get_categories($args); //sets up everything to be looped through

foreach ($cats as $key => $category) { // loops through categories in array and creates a section for each

$catID = $category->cat_ID; //sets catID to category ID number
$catName = $category->cat_name; // sets catName to category Name
$cat = $cats[$key]; // Sets category

and then echo it later-on in my code,
and with wp 2.7.4 it worked, with wp 2.8 it just returns "blank".

any clues much appreciated,
gerne auch in deutsch.

I was putting together a custom feature for a client to automatically post some stuff, and I needed it to create categories if they didnt exist - I figured I'd just loop through the desired categories and check if each of them exists (with category_exists), then create them if necessary - but then I found another function to do it for me -
function wp_create_categories($categories, $post_id = '') - it loops through the $categories array and creates the categories if they dont exist, and then applies them to post $post_id (if supplied).

Anyway, I never would have found it if it werent for this post - Thanks!