macOS guest gradually (!) stopped booting on upgrade

When I upgraded from a (much) older version of Parallels to 15, I had several virtual machines, including one older version of macOS (10.10 I believe) for testing. I used it repeatedly since then, but it often failed to boot, pretty much at random. I type my password, it would start with the progress bar, which would slow to a crawl and eventually stop, and then if I left it long enough it would show some kind of error screen. But if I tried again a few times, it would eventually successfully boot. And yes, that seems weird to me too, especially since I always take it back to a known snapshot so this can't have anything to do with disk state.

I eventually turned on verbose boot, so I could see where it was getting stuck. The last few lines before it just stops dead are, every time:

MAC Framework successfully initialized
using 16384 buffer headers and 10240 cluster IO buffer heders
AppleKeyStore starting (BUILT: Sep 1 2015 20:56:51)
IOAPIC: Version 0x02 Vectors 64:87
ACPI: sleep states S3 S4 S5

And there it sits, doing nothing, forever.

And then again, occasionally it powers right past that in way less than a second and goes on to boot successfully.

Hooray for non-deterministic behavior!