There seems to be a conflict with the newest version of Woocommerce (3.2.1) When using a coupon that has no limits set per coupon (Usage limit per coupon: Unlimited usage), the customer gets the message after trying to confirm payment that “coupon usage has been reached”. This issue goes away when reverted back to the prior version of Woocommerce (3.1.2).
The site is in the middle of a promotion right now that is using the affected coupons, so I can’t upgrade Woocommerce and do any testing at the moment or it will interrupt that. If I have time, I’ll clone the site and then give it a try and report back.