Hello again,
This is an excerpt from the answer I've got from Aprelium minutes after I've sent the requested configuration file:
"You are getting error 403 when the click is made from an outside link because you have activated antileeching on all pages served by
www.mysite.com.
So if you expect to have people visit you from external links, we suggest that you disable antileeching on that Web site or at least restrict it on certain paths (paths that are not meant to be linked to directly from Google or from external ads.)"
I didn't realize that antileeching was causing all the trouble, but I'm very happy it works properly now.
Thank You! :)