When a moderator hides a forum topic (and all the comments), s/he winds up 'Unpublishing' it. This means when the page reloads s/he receives a 403: Access Denied from the Web site- after all, the Drupal node they were just viewing is not no longer published and they do not have permissions to view it.
The quick solution is to change the redirect after the hide command to go back to the forum page. The other redirects (edit, lock, sticky, etc.) can remain the same.