Ok
I was checking the apache error log and I found this:
PHP Fatal error: Call to undefined function curl_init() in /var/www/teste/wp-content/plugins/ajax-for-all/inc/admin.php on line 229
I installed php5-curl library:
sudo apt-get install php5-curl
and restarted apache server
and now it works!!!
My recomendation for you is to explain that php5-curl library must be installed for this to work.