What is the rule at this traffic-light-controlled junction with additional signage?

Your answer is correct. When you see this additional sign at a red light, it means that only cyclists are allowed to turn right even when the traffic light is red, provided they proceed carefully and yield to other traffic and pedestrians. Motor vehicles are not allowed to turn right, and cyclists are not required to turn right—they just have the option to do so. This is a special rule in Germany to improve traffic flow for cyclists. Recognizing such signs helps you understand permissions for specific road users at intersections.