Having an issue that is very repeatable. When wandering inside Science Labs, after descending a floor or two (usually whatever floor Mi-go are on) I get a debug message stating, if I recall correctly, “could not find monster by type”. The debug isn’t clear-able by pressing any keys, so it basically causes me to Alt-F4.
I’d reload before the Lab, and run through again, and crash at the same spot even with a freshly generated floorplan.