So I tried on a whim, and I was miraculous able to play again today after two weeks of nothing but crashes.
But, after playing a while, I got the witch, and when I upgraded her, this happened: Not sure what was going on here, but I couldn't scroll through the whole stacktrace, so what you see is what you get. :/
It looks like it might have been trying to save right when I upgraded and that screwed up via some kind of concurrency issue, but that's a pseudo-random guess.
Edit: I say major because I was unable to exit this dialog, so the game effectively crashed.