Cristian Lavaque on "[Plugin: s2Member Framework (Member Roles, Capabilities, Membership, PayPal Members)] Return from PayPal White Screen"

Maybe. If you haven't yet, you could try these: http://www.s2member.com/kb/common-troubleshooting-tips/

But I'd also make sure the restriction is done correctly. WP Admin -> s2Member -> Restrictions

And remember that you need to set a Membership Options Page for the restrictions to become active. WP Admin -> s2Member -> General -> Membership Options Page