Coupon codes conflict

Hi,
I have an ongoing discounts code conflict on my site which we cannot solve with my IT team.

Issue: in cart, buyer gets error notice: “coupon code already applied”, despite the coupon box is empty. And result: you cannot finalize the purchase.

It’s not happening all the time, but occasionally with some customers, so it’s difficult to understand what’s causing that code conflict.

I have WooRewards, Advanced coupon codes in use on my site – we have tried everything, but haven’t been able to solve the issue.

Could someone smart help to solve it please…

The page I need help with: [log in to see the link]