Hello Christoph,
I would guess that your SAML Authentication between SuccessFactors and your HCP Account is not correctly configured. I have used the SuccessFactors Role Provider before without any problem.
Did you follow all the steps in the SAP Hana Cloud Platform - Extension Package for SuccessFactors Guide to configure Authentication/Destinations between SF and HCP?
You can find it here: Foundation – SAP Help Portal Page
Best Regards,
André Bezerra