What is the right behaviour now?

In this situation, the correct action is to stop behind the taxi. When another vehicle, like the taxi here, is changing lanes and moves into your lane ahead of you, German traffic rules require you to give way and ensure you maintain a safe following distance. Passing the taxi on the right or quickly moving into the left lane to overtake are both unsafe and not allowed in this scenario. Stopping or slowing down behind the taxi allows for a safe traffic flow and prevents accidents. Always prioritize safety and give way when another vehicle merges into your lane.