I have tested with this and not work:
$thumb_url = 'http://s.wordpress.com/mshots/v1/clarin.com?w=400';
And test with this and it works:
$thumb_url = 'http://i1.ytimg.com/vi/ZWaX1g_2SSQ/default.jpg';
1. What can be happening?
Another doubt it that the code not work with the plugin WPUF but works with wordpress backend. I think that the plugin WPUF not uses the action "save_post".
2. Exist a way to add this action to the plugin, to test if works?
Thanks and greetings