The vehicles ahead of you are driving at about 30 km/h. Why are you not permitted to overtake before the road bend?

Your choices 1 and 2 are correct.

Answer 1 (“my overtaking distance is too long”): At only about 30 km/h it might feel easy to pass, but you still need enough clear road to pull out, pass the vehicle(s), and move back in safely. Before a bend your view ahead is limited, so you cannot be sure the opposite lane will stay free for the whole manoeuvre. If you start overtaking and then the bend reveals oncoming traffic, you may not have enough distance to finish and return without forcing someone to brake.

Answer 2 (“the red car might pull out to overtake”): When several vehicles are driving in a line, a vehicle further ahead may also decide to overtake. If you start overtaking from behind at the same time, you risk a side collision because the driver ahead may not expect someone behind them to pull out (or may not see you in time), especially near a bend where everyone’s attention is on the curve and visibility is reduced. Rule of thumb: don’t overtake into a “hidden” area and don’t overtake in a queue unless you can clearly rule out that someone ahead will pull out.

In general, overtaking is only allowed when the traffic situation is completely clear (“unklare Verkehrslage” means no overtaking). Limited sight due to a bend and the possibility of another vehicle pulling out are classic reasons why the situation is not clear.