What is the right behaviour now?

In this situation, the traffic rule is that you must accelerate when appropriate—especially after passing a construction zone with a speed restriction once the restriction has ended and the road is clear. The correct answer is to accelerate, because this is typically expected when leaving a temporary lower speed limit, for example after passing through a construction zone. Unless there is a sign still indicating a speed limit, or another hazard requiring you to stay slow, you should return to the normal speed for the road.

The other answer about accelerating only when the speed limit is removed by a road sign would not always apply, because sometimes you might not see a specific end-of-speed-limit sign—German traffic law also recognizes situations where the restriction ends naturally, such as after you have clearly left a temporary construction area.

So, the rule to remember is: once you are past the construction site and there are no more signs restricting your speed, it is correct and expected to accelerate back to the usual speed for that area. This shows that you understood both the temporary restriction and the normal flow of traffic.