hey @maja.effenberg it works for me.
if it gives 409, that means you have to check in the console if the role exists in the Project, everything's in the screenshot. You have said earlier "one of the defined Roles from one of our projects." can you please cross check/verify if that's the case, make sure you are sending your ROLES KEY and NOT the Display Name.
In my case, role key is learner and display name is LEARNER.