In order to fix the problem you need to update the redirect setting for the server validation on the contact_basic.php page.
By default the form redirects to itself, which in this case redirects to the plugin and not the page.
Update line 5 of contact_basic.php to from:
$WAFV_Redirect = "contact_basic.php?invalid=true";
$WAFV_Redirect = "" . $_SERVER["PHP_SELF"] . "?invalid=true";
That should fix the problem. We'll probably have to update form builder's server validation to use that code automatically so that it will work better with plugins by default in the future.