I still have not solved the problem… apparently the bodyinstance (physics) is somehow corrupted or invalid or something…
I have put the code to check:
if (mBody && mBody->IsValidBodyInstance()) but it still crashed.
This time I think this is probably due to me upgrading the project from 4.19 → 4.27 and cause all kind of weird issues. So now, I will try to create a new vehicle bp derived from fgear and have it as simple as possible - hopefully this will not crash. And then, I will add back old code slowly…