As said before, I see no obvious reason for the fail in your code. Are there further changes in the posting.inc.php?

F.Y.I.: I found the reason. It was a typo in posting.inc.php. I modified to

if (isset($_POST['tags']) && isset($_SESSION[$settings['session_prefix'] . 'user_type']) && ($_SESSION[$settings['session_prefix'] . 'user_type'] = 0)) {

instead

if (isset($_POST['tags']) && isset($_SESSION[$settings['session_prefix'] . 'user_type']) && ($_SESSION[$settings['session_prefix'] . 'user_type'] >= 0)) {

Note the missing '>' in the above expression.

Now, I have both: admin edit AND tags for simple users

N.