Thanks fot your advice, I tried to rename the plugin directory, so it doesn't work, but there was no difference in the results.
Moreover, in these last two days, all the three sites I built with WP MU have problems: only the main site is visible, and the secondary ones are horrible, because the css and everything else isn't working (only rough text is visible in http://ilcavallobianco.com/www/en/...).
I checked both index and htaccess.
And that's not all! At the moment, I mean today, in each site I had to use a simple php forwarder, because using the right WP index made also the main site not visible, with "Error in establishing DB connection".
On the countrary, yesterday I was forced to use the WP index, for the same reason.
It looks like there were a ghost...
Do you think that reinstall everything could resolve?
I cannot understand why everything worked well for 12 month and all of a sudden ruined, without I made changes. What could be the matter? Any idea? Thanks for your help.