404 Error after subscribe to newsletter

Hi there,

I am just trying to implement the e-newsletter plugin on one of my sites and seem to have managed to do most of the set-up.

I have added the signup widget in my footer and it adds a new subscriber as you would expect, but after the subscription, it redirects you to an error 404 page with the following URL:-
http://www.mysite.com/admin.php?page=newsletters-members&updated=true&dmsg=ThenewMemberwasadded!

Which doesn't look very professional. Why would this be happening? Am I missing something in the setup of the plugin?

e-newsletter - 1.0.3
WordPress - 3.2.1
Headway Theme - 2.0.11

Hope someone can help.
Regards,
Craig