Sorry for the late reply. I believe the problem is that for a coupon to be applied for a guest user, there must be something added to the cart before that, i.e. coupons can not be applied to an empty cart for guests. I will have to find a solution to this – will get back to you asap. Meanwhile, it looks like you can “solve” this by adding some product via URL at the same time when applying the coupon. For example:
This is now fixed in the latest plugin v1.2.3. To make the coupons work for the guests with an empty cart, you need to enable the new “Force session start” checkbox in plugin’s settings (in “WooCommerce > Settings > URL Coupons”).
Please give it a try and let me know if there are still any issues.