1. No, Terrasync does not work because of
this issue, so I am not using it. It did work earlier on FlightGear 2.8, but not since I upgraded to 2.12.
2. The first time I tried multiplayer was a few weeks ago, and the location was completely different (IIRC I was flying from Santiago de Compostela to Faro, when FlightGear crashed near Coimbra). I thought then that it was just a fluke. But that makes still it 4 out of 4 attempts.
3. The three last times mpserver12, the first I don't remember,
Incidentally, I was flying a DC-3 in all attempts, but that should not be an issue as I have been flying a kind of European tour (not connected), and while the program has crashed two or three times, it usually it works just and I have been able to finish 3+ hour flights, including visiting such developed airports as LOWI or EDDS. The scenery has been the same since Terrasync has not been on. The scenery is not the new 2.0.
I do not think that the groundnet errors have much to do with it, since they are completely routine; I see them every time.
It may be that the problem has nothing to do with multiplayer, but as far as I can tell that is the only thing that is different. I flew the same EFKU-EFMA route last weekend unconnected, with the same settings, and encountered no problems.
Well, maybe I try some other route, let's see what happens.