I've come to this late, but from using the test at the URL you gave it looks like you haven't sorted it. looking at the source you need to put either a relative or absolute path to the script in the action parameter of the form.
e.g. action="/cgi-bin/feedback.php" or action="../cgi-bin/feedback.php
Other things to check are permissions on the feedback.php file and your sendmail path.
Hope that helps.