Thanks for all your support. In the meantime, we solved our problem by creating an external RADIUS server. Basically, everything works now.
However, we have another problem: The service controller causes a lot of network traffic to the (currently only) access point AND even disconnects our firewall from the network!
The MSM760 and one node of our firewall cluster (Fortinet) are connected to the same switch (Procurve 4208vl). The other firewall node is connected to a Procurve 4108gl; both switches are connected of course.
Now, when the MSM760 is connected to the switch, after some minutes we lose our external connectivity and can't even ping the firewall anymore. All other stations can be pinged. That happens every few minutes, and after some minutes, connectivity comes back. When we disconnect the MSM760 from the network, everything works perfectly again.