Nonsense. The itinerary is not a one way but an open-jaw return. BER-VIE/VIE-LHR. This is allowed because the open sector LHR-BER is shorter than VIE-LHR. The multi-city option prices it correctly but the booking engine behind it fails to establish availability.

Also, stopovers are in principle allowed on one-ways but usually in higher fare tariffs and often with a stopover fee meaning two one-ways may be cheaper. An open-jaw return prices normally significantly cheaper than two one-ways.