I tried for a long time but found nothing. I walked 30 floors with your safe, without crashing, can't replicate it. I check the art files and also the enemy encounter in which the crash happend.
My guess is that it could be a bug that only happens with older safe files that are carried over from version to version. But I can't replicate it, so no idea, just a loose theory.
I also fixed a another bug with kitsune forms yesterday, so maybe that randomly fixed it, and cause of that i can't replicate it? But I highly doubt that.
So until more information about this crash comes to light I need to assume it is a hiccup from using older safe files. If that would be the case a it wouldn't happen in new game or ng+.