When driving behind a vehicle that is travelling at a speed of 80 km/h on a road marked in this manner, is it mandatory to keep a minimum distance of at least 40 meters from it?

Yes. On a motorway, the gap in metres must be at least half your speed in km/h. Following at 80 km/h therefore requires at least 40 m. This specific rule does not apply during overtaking; the distance must always be safe for the conditions.