Ntdll.dll crashes are usually caused by OOM, usually because your sim was too loaded of too many addons at the same time, with the most consuming one usually being a memory hungry airplane.
So, the sim crashes because it ended its available memory, so this MADE Couatl crash (Couatl cannot crash the sim, since it's an external .exe), because it didn't get the signal from FSX to close properly, since FSX crashed before being able to sent it, so you see a crash related to Couatl.exe, but it's a victim of the problem, not the cause.
Try the same with a default airplane.
