WooCommerce URL coupons v2.4.1 has a bug where it does not redirect from a coupon page properly if the site is using https. We have submitted a ticket to support and hopefully this will be fixed asap.
The problem is in /includes/frontend/class-wc-url-coupons-frontend.php L106:
$url = 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
A quick hacky fix for sites running exclusively on SSL would be simple:
$url = 'https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];