Had the misfortune of needing to rebuild a Windows 10 Pro machine. The only install DVD that I has was from July 2016 – so almost 4 years old.
Install went fine, but was left with the install at build 1511. But for the life of me, couldn’t get it to update.
There are a bundle of random pages on the Internets, like https://answers.microsoft.com/en-us/windows/forum/windows_10-update/windows-10-wont-update-error-code-0x80240fff/4fe63cab-0bef-4093-bad1-650c779dbbd0
But none of the suggested fixes worked. One thing that did ring true was that this may occur “when trying to update from old builds”.
Solution: Run the Windows Upgrade Assistant: https://support.microsoft.com/en-nz/help/3159635/windows-10-update-assistant
This manually pushed the machine to the December 2019 build and then updates ran fine after that.