lorro— implemented your instruction of, at Admin page / Settings / General / ‘WordPress Address (URL)’ and ‘Site Address (URL)’ should be the same. In my case the the Site Address URL did not end with /wp as does my WordPress Address.
So, now the ‘Cart’ does update and the ‘Proceed To Checkout’ does advance to checkout.
However, ‘Sales Tax’ still does not display or calculate.
Much thanks, so far.