hmm, I have after you submit a new Main Cat or New Sub Cat a redirect to $WA_redirectURL = "MainAndSubCat.php";
Try remove the redirect or redirect to a empty page with a back link, see if that solve the forbidden error.
I did the redirect so you could see the changes direct after inserting a new Category. :)
Remove: action="<?php echo $editFormAction; ?>"
I think its a bit of left over code from an alterd insert of a modyfied field in Form Builder. :)