This: "I get an error (-6000 -83 from memory) when saving the backup to the desktop. No method I can find works."

-6XXX errors are related to the Windows Username access rights.

This: "If it is relevant, I am running Quickbooks in a Windows 10 virtual machine."

Is the Most Relevant thing you have stated all this time. You have a Windows Rights issue, and Windows 10 manages user rights differently. Make sure to Install QB "for all users" and "run as Admin."