[stylin19a]

I've just been running Linux under VirtualBox on Windows

Were you running the above when you first upgraded or first installed of Windows 10 ?

Unless i missed it somewhere in this thread, I'm thinking you probably should be looking at the Windows\Virtualbox. Be kind. I mention out of ignorance.
I don't know enough about VirtualBox\Linux.
Can VM\Linux be turned off so you can run just windows and then do the upgrade ?
I'm guessing the reboots might be getting whacked during the upgrade.
Is there an upgrade version of VirtualBox that may need to be installed\upgraded because of Windows 10 Creator upgrade ?

list of upgrade error codes:

https://docs.microsoft.com/en-us/windows/deployment/upgrade/resolve-windows-10-upgrade-errors#0x800xxxxx

Good Luck...keep us posted. Let us know the solution.

[Pearls Before Swine]

Were you running the above (i.e., Virtual Box/Linux) when you first upgraded or first installed of Windows 10 ?

No. I shut down all unnecessary programs when doing the update.

Thanks for the upgrade error link. The things it wants me to do seem fairly complex, but I'll look into them. Particularly, the comments about profiles.

As this is my primary machine, I'm not sure I want to go there, since the rollback version works just fine.