BuddyPress.org

Thanks…it worked.. solution is in https://buddypress.org/forums/topic.php?id=302

We solved the problem commenting the following two lines in the .htaccess file of WPMU root:

#RewriteCond %{REQUEST_URI} !.*wp-content/plugins.*

#RewriteRule ^(.*/)?files/(.*) wp-content/blogs.php?file=$2 [L]

Thank you Diego!