Hi Alessandro,
No problem, let’s try it this way. I’ve imported the search instance, and found only one possible issue within the configuration. On the Frontend Search settings -> Post tags panel the tag filters are enabled, but not used. Can you please turn them off?: http://i.imgur.com/m9fjcol.png
My guess is that there might be a conflict whenever this filter is turned on, but the settings are hidden. Let me know if that changes anything.