Hi Stefano,
I found the reason of the error in the unsuscribe procedure. As suggested by “Florox” here, I added a return statement (return $user;) at the end of the unsuscribe method (line 669 of plugin/newsletter/subscription/subscription.php) and now it work correctly.
Please include this small bugfix (or an equivalent one) in the unsuscribe method in the next release, so it will be available to all users.
Thank you.