Fatal error on update

Hi! I’m using WordPress since 2 weeks and everything went fine… until today. This is what happened.

I clicked on the Automatic Update button to install version 3.3.2.
At the end of the update process, a “500 Internal Server Error” displayed.
Searching on Google gave me the answer: the problem is with my italian server provider, Aruba. So I recovered the permissions and inserted these 2 lines in wp-config.php:

define(‘FS_CHMOD_FILE’,0755);
define(‘FS_CHMOD_DIR’,0755);

FInally I was able to log in back to my site: the version was updated, 3.3.2.
So I tried reinstalling the update, to see if the Internal Server Error problem was happening again and, clicking on the Automatic Update button gave me this message:

Fatal error: Cannot redeclare class Plugin_Upgrader in /web/htdocs/XXX/home/wp-admin/includes/class-wp-upgrader.php on line 631

Now I really don’t know what to do. After searching of forums, I’ve disabled the plugins but the message is still there. What can I do to bring back the automatic updates?

Thank you very much. Bye!