Which one is blocked? From muzz' post, I have the impression that if you have both a homepage category listing and a middlebox activated, the middlebox is the one that doesn't show.
I don't know what files are included in the mod; if one of them is /includes/templates/your_template/templates/tpl_index_default.php, that file would be replaced when categories are active by tpl_index_categories.php.