Class ‘WP_Widget_Factory’ not found

Dear all,
Even if I am not new to WP upgrade, I am cannot sort a problem out this time.
I followed, as usual, all the istructions for upgrading, however the following is the message that I get:

Class ‘WP_Widget_Factory’ not found in /web/htdocs/www.intransito.biz/home/blog/wp-settings.php on line 625

I do not know what to do. The strange thing is that my blog’s path is http://www.intransito.biz/blog and not http://www.intransito.biz/home/blog

Please, is there anybody able to help me?

Cheers,
Fopy