Drupal 7: Can't Update node using php script
Make sure you're logged into the site as a user with permissions to update the specified content type, because that session will carry over to your bootstrapped script. If you're logged out, you're essentially attempting to update the node as an anonymous user in your script, and permissions will come into play.

Try adding...

global $user;
print_r($user);


...to your script and see what, if anything, that outputs.