In branch '3-0-stable'(Spree), the Add to Cart button redirects to a page which doesn't exist, without adding the product to the cart. It redirect to orders/populate path but then:
This is because there is an error occurred. And your setting might not allowing throwing exception in front. Maybe because this is not in development environment.
Anyway, order/populate
path does exist on 3-0-stable
Spree. Please look in here and here.