I have not seen anyone suggest this so I'll give it a go.

If you're having problems upgrading, try enabling the hidden Administrator account, sign out of your normal account, sign in as Administrator then try the upgrade.

Open a Elevated Command Prompt.
Type net user administrator /active:yes and press Ester.
Exit Command Prompt.
Sign in as Administrator.
Insert flash drive or DVD or mount the Windows 10 upgrade ISO.
Run setup.exe from any of the above.

I suspect this would only make a difference if there is some kind of account corruption, but what the heck, it's worth a try.