[Plugin: Constant Contact for WordPress] Submitting Constant Contact sidebar widget returns 404 page

WP 3.3.2 – Constant Contact API 2.3.9 – After encountering problems with saving styles in Form Designer, I configured the Constant Contact Signup Widget , and added it to a sidebar. When I submit a registration, it returns a 404 error page with a sidebar widget message “Success, you have been subscribed.” I searched plugin options and documentation, but could not find any way to specify the page to be returned to the user upon successful submission of the widget form. It seems that it should display the same page from which the form was submitted. Please advise how to redirect to a page other than a 404 when accepting submissions from the widget.

http://wordpress.org/extend/plugins/constant-contact-api/