sounds like you have 2 different login pages.
what is the address when you login the first time?
What is the address on the login page you are redirected to by the checkout page? it sounds to me like like these are not the same login page.
to determine the shipping problem, open the failure page, on the bindings panel, expand the eCart binding group.
there will be a binding for the shipping providers full request and full response, add these to the page.
NOTE: the full request and full response will contain sensitive information that should be removed before posting here.