Since we upgraded our VCSA to 6.0u1 we can no longer use the "Use Windows session authentication" to authenticate to the VCSA. I need to use the notation domain\username to make it work? Its an embedded deployed and I#'ve checked that its joined to the domain.
Take a look at this KB article for some troubleshooting tips: VMware KB: Unable to login using the Use Windows Session Credentials feature in the vSphere Web Clie...
So it looks like this KB is the closest match,
* VMware KB: Logging in to the vCenter Server with Use Sessions Credentials fails with the error: User name and password are required (2112731)
Going to test this one further and will let you know.