I'm happy this helped someone else.
I tried to do a search on my blog, nad everything works fine, in both Italian and English.
I get a "page not found" error only when I try to change language clicking on the language widget from the search results page, but I thinks this is expected (that search was for posts in the other language).
@Malesandro, could be something wrong in your server configuration?
On my blog, search urls are like this:
http://gerloseye.org/en?s=lorem+ipsum
In your URL there seems to be a slash between the language url and the query string.