Hi again Stefano:
Since I still need to solve this I did further experimentation and I discovered what is causing this problem.
First, I tried a number of different themes. That didn't help the problem.
THEN, I disabled SSL on the site. Bingo! That solved the problem.
So now the question is, why is SSL causing this problem? I tried looking at your code in subscription.php and I found the two lines where the code echoes "wrong email" to the screen. Unfortunately, though, I don't know enough about php to know how to solve the problem. The two locations that I found were:
1. line 115:
if ($email == null) die('Wrong email');
2. line 303:
if (!$newsletter->is_email($_REQUEST['ne'])) die('Wrong email address.');
I hope you can tell what's needed to fix this.