I have another clue,
I have taken a look on apache error.log and there is a php notice like this:
[Tue Dec 10 16:01:57 2013] [error] [client 90.165.31.215] PHP Notice: Undefined index: CONTENT_TYPE in /var/www/santander/proxy.php on line 107
[Tue Dec 10 16:01:57 2013] [error] [client 90.165.31.215] PHP Notice: Undefined index: HTTP_REFERER in /var/www/santander/proxy.php on line 108
Please, any idea?