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

Your selected options are correct. According to German traffic rules, you are not allowed to enter a junction even if the traffic light is green in certain situations.

1. If you would have to stop in the middle of the junction because traffic is backing up, this is to prevent blocking the intersection and impeding the flow of traffic, which is mandated by the rule of not obstructing traffic.

2. If a police officer instructs you to stop, you must always comply with their directions, regardless of traffic signals.

These rules are in place to ensure safety and maintain smooth traffic flow, preventing potential accidents at intersections. Always ensure you are clear to enter a junction before proceeding, even with a green light.