No idea why it would give an error (by the way, I just changed the Newsletter_Email class and Page class, removed the extra forward slash in the ViewNewsletter/ link, the one before ViewNewsletter [ie. old = /ViewNewsletter/; new = ViewNewsletter). It hasnt given an error for me. What version of the newsletter module are you using? Are you in 2.4 or 2.3?
Does firebug give you an error? try using the firebug add-on for firefox, and when you send it should have some error details show up if you have it enabled.
I also added another change for a the NewsletterEmailProcess.php class, check previous post for changes.
In NewsletterEmailProcess.php replace the following line:
$e = new Newsletter_Email($this->nlType);
$e = new Newsletter_Email($this->nlType, $this->newsletter);
This might fix your problem, but I'm not sure...
NOTE: If you will use the extend subscription page option, yes it's better to remove it the code from the Page_Controller class, and add it to your new Class controller