Yes, the plugins are all up to date.
I've already opened a ticket towards the smart coupons developers.
What I've noticed is that if I install your competitor's (payre....) plugin instead of wallee, the behavior is the same. When the cart goes to 0 I get an error.
So most likely the issue should be fixed by cross testing the 2 plugins together.
I'm not sure I can help too much, this looks like a plugin compatibility issue.
Hope you have a chance to download the plugin and test. Otherwise please let me know which "shop credit" or "coupon" plugin I can use so that I can sell vouchers online.
To replicate the issue:
- create a "store credit"
- purchase products for less than your "store credit"
- cart total will be 0 and then try to pay