Cannot upgrade Windows 8.1 to Windows 10

I am finally getting round to upgrading my Windows 8.1 guest OS to Windows 10.

I've purchased a licence for Windows 10, and have followed the instructions in this knowledgebase article - https://kb.parallels.com/en/122884. I have plenty of disk space and my Windows VM has plenty of memory.

The first time I ran the upgrade tool, it failed with the error:
Couldn't install Windows 10
0xC1900101 - 0x20017
The installation failed in the SAFE_OS phase with an error during BOOT operation

Subsequent attempts result in no error, the upgrade tool just quits. I haven't managed to see at which point it quits (I can't sit staring at a progress counter for 2 hours!).

I believe the log files for the upgrade tool are here on the Windows VM:

C:\Windows\Panther\

Unfortunately I cannot see any useful log files there. the setuperr.log file is empty, and the setupact.log file doesn't appear to have any glaring errors in it.

Any pointers please?

Regards,

James