I remembered something about VirtualBox not playing nicely with Hyper-V on Windows, and dug up a possibly relevant post[0] on their forums. IIRC we ended up moving a few build systems to Docker and dropping VirtualBox because of hyper-v related issues, but it's been a few years.
>>gopher+(OP)
That's the unrelated green-turtle issue. It's only relevant after the guest has actually started running instructions. I'm talking about before that point.
>>datafl+r
I'm not aware of any turtles, that was just the first thing I found when trying to see if VirtualBox and Hyper-V were still a problematic combo.
Again, it was a few years ago, but we didn't solve the problem or identify an actual root cause. We stopped banging our heads against that particular wall and switched technologies.