[@project_subdomain Participant 11 years, 3 months ago]

thanks, robkk, you are right.

found a solution that may be useful for some, though:
It’s these three plugins together:
Paid MembershipsPro + Paid Memberships Pro bbpress + Paid Memberships Pro: PMPro Roles
Tried, worked, but unfortunately slowed down my page already and found it to be a bit too much overload for what I need.