I might have an answer to this. After searching on the internet I found some others who had similar issues, one of whom posted a log that (apparently) showed their VM trying unsuccessfully to access a flash drive just before the crash. For what reason is anyone's guess.
Anyway, this made me wonder if my problem started when I began using an SD card on my host machine. So I took it out and re-tried a recent update that had crashed the VM on first attempt (it was consistently the monthly quality rollups that broke it). It worked.
The SD card slot on my host machine (and likely most/all others) connects via the USB controller, which means that a USB stick drive would probably cause the same thing. The solution to it would appear to be unchecking the "Automatically connect new USB devices" option in the VM settings so the VM doesn't pick up any of the host's USB drives on startup.