When trying to add a product to cart from an external domain the site gives error. The add to cart url looks like this on the external site which has a button with the link below:
The page where this button is has the URL https://externalsite/store/1730319
internal path component 'https://externalsite.com/store/1730319' is invalid. Its path component must have a leading slash.
However, when the link "cart/add/p[nid] is triggered from the same domain site it successfully adds the product to the cart.
Ubercart D8 version : 8.x-4.0-alpha5+227-dev