[Plugin: WooCommerce] "Please select a value that is no more than 0" message

On the Cart page when I want to apply a coupon or update the cart, a message appears next to the quantity field of variable products reading, “Please select a value that is no more than 0”.

However, when I proceed to checkout without clicking on “Apply Coupon” or “Update Cart” there is no problem at all. But when I click on one of those buttons, I get the message above.

I already checked the inventory options of the products. All seems fine. The inventory is definitely larger than zero.

Any help is highly appreciated! This problem is driving me crazy and keeping me from opening my shop (where I wanted to grant people a coupon).

Thanks very much for your help!!