Ok, I found a workaround:
Everytime you fly in MP, a track gets saved by default by DCS. So you can fly without Tacview and THEN after the flight, watch the track with tacview enabled and you'll get the same file as you would with tacivew on the flight. The only downside is that you'll have to replay the whole thing again. However, you can speed up the thing and for a 2 hour flight you'd have to wait like 20 min to generate the tacview file!
EDIT: I compared the .acmi form the last flight night vs the track of the FN and its the same thing. That is a 2.5 hr flight watched in less that 10 min (time x12).