We're integrating our portals (Salesforce & External portals) and want to deliver content inside Salesforce to the external portal.

So we've setup SAML SSO to provide a seamless data access.

However, I've just checked to ensure that our users (customer & partner portal users) can still use the traditional login URLs to gain access to the systems and not been able to login.

(a la: https://tapp0.salesforce.com/secur/login_portal.jsp?orgId=xyz&portalId=xyz)

The error returned is "Your company's Single Sign-On configuration is invalid. Please contact the administrator at your company for more information."

How do we configure Salesforce to allow both SAML login AND "traditional" login?

What does this mean?