In your comments.php file of your theme, try changing $cookiehash to COOKIEHASH as below.
From:
if ($_COOKIE['wp-postpass_'.$cookiehash] != $post->post_password) { // and it doesn't match the cookie?>
To:
if ($_COOKIE['wp-postpass_' . COOKIEHASH] != $post->post_password) { // and it doesn't match the cookie?>
