Which vehicles have to maintain such a great distance between them and the vehicle ahead when not inside a built-up area, that an overtaking vehicle can pull back into lane?

You answered correctly. Outside built-up areas in Germany, trucks with a gross vehicle weight over 3.5 tons and combination vehicles longer than 7 meters must keep enough distance from the vehicle in front so that an overtaking vehicle can safely move back into the lane. This rule helps ensure overtaking remains safe for all road users. Trucks up to 3.5 tons do not have this specific requirement. If you see such vehicles on highways or country roads, expect them to leave large gaps between each other. This is an important safety rule to help prevent accidents during overtaking.