Well, I’m sure I’ve found the cause: mon_null. Whenever the game tries to create (or maybe assign a variable like is_animal to) a mon_null (the default for many, many monster groups), it crashes.
Not a solution to the crash, but at least I (and everyone here) now know what causes it…
Sadly, I’m too exhausted to code up a solution for that right now. But I’m sure someone will pick this up and fix it till tomorrow (or I’ll do it when I get up).