Error saving settings for shipping and blank checkout
I'm testing a fresh install of Magento 2.2.6 (CE). When I try to change settings for shipping methods, I can't save! Everytime that I try to save I got this message: "Magento Shipping authentication failed. Please check your credentials.".
Magento Shipping module is NOT ENABLED. I didn't filled any information in this module, it has default values. I've installed another modules for "Correios" and this is active, but I don't thing it's related. Even disabling Correios I stil can't save anything in shipping methos, always this error message.
Another problem is with checkout page. With multiple shipping enabled (default) and one page checkout enabled (default too), when user click on "Checkout" in cart, I got a blank page. Well, not totally blank....page has header and footer (from store), but no content.
If I disabled One Page checkout (but keep Multiship enabled), I can got to cart and click on Multiship to finalize my order.
If I disable OnePage and Multiship, I can access my cart, but there is no button to finalize my order.
Not that this user (test user) is a fresh new too, no address yet (New User registration during checkout).