Same issue, was doing an empty() check on $page_urlname in Admin:PageManager and not taking both main pages into account
--- a/plugins/admin/PageManager.php Sat Sep 26 15:21:51 2009 -0400
+++ b/plugins/admin/PageManager.php Sat Sep 26 15:26:14 2009 -0400
@@ -150,7 +150,7 @@
// Field: page URL string
$page_urlname = $_POST['page_urlname'];
$page_urlname = trim($_POST['page_urlname']);
- if ( empty($page_urlname) && get_main_page() !== '' )
+ if ( empty($page_urlname) && !have_blank_urlname_page() )
{
$errors[] = $lang->get('acppm_err_invalid_url_string');
}