In which situations are you not allowed to enter a junction even though the traffic light is green?

You answered correctly. Even when the traffic light is green, you may not enter a junction if you would have to stop in the middle and block the intersection because of backed up traffic. This prevents gridlock and is always required under German traffic rules. Additionally, if a police officer instructs you to stop, their directions override the traffic lights and you must obey them. However, a "Stop. Give way" sign must only be followed when the light is out or not operating; as long as the traffic light is functioning, the light takes precedence over the sign. Remember, police orders always override signals and signs, and never block intersections even if your light is green.