Why is the plugin redirecting users to “Cart is empty” page after they order?

hi we have purchased your product “order minimum/maximum amount for woo commerce”.  Works okay but for some reason when we reach the checkout stage, and press “send order”, instead of directing to order details,  it directs to “cart is empty”.  Though it seems the order is complete like we receive confirmation order.

Took us time to figure this out, we have deactivated other plugins and still the error points to this plugin.

Let us know what we are missing.  thanks.