You are driving in dense fog on a federal road (Bundesstraße) outside a built-up area. Are you allowed to drive in the middle of the road continuously straddling the broken white line?

You answered correctly. Continuously driving in the middle of the road, straddling the broken white line, is not allowed—even in dense fog. This behavior is dangerous because it increases the risk of a head-on collision with oncoming traffic. Regardless of visibility conditions, you must stay on your side of the road and only cross the center line when overtaking, turning, or avoiding an obstacle, and only when it is safe. Using fog lights improves visibility but does not give permission to drive across lane markings. In summary, always keep to your lane to ensure the safety of all road users, especially in poor visibility conditions like fog.