WCCP users should be redirected to the SSL Login page through Smoothwall's WCCP transparent interface.
When users use WCCP and have their Smoothwall WCCP transparent interface set to redirect to the SSL Login page, the SSL Login page redirects via a redirect to the hostname.
Any issues with this will be due to missing DNS entries for the Smoothwall hostname.
It is a WCCP requirement that the client machines be able to properly resolve the Smoothwall hostname via DNS.
Check your DNS entries are up to date.