I'm having problems too
I checked all the code, and the redirect path is correct. If i open up the cart page manually, the cart is still empty, but if i then click 'add to cart' again, the item is added.
Could just be me as until now i haven;t seen this error reported.
I have just uninstalled the extension and reinstalled version 4.1.6.
Will update when I have had a go with this.