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... and that is exactly the reason why I have posted this warning at the beginning of the tutorial:
WarningNotice please: When Windows Update has finished downloading the UUP upgrade, the files needed to make the ISO are stored in a subfolder in C:\Windows\SoftwareDistribution\Download.
These files will in some cases be removed when upgrade is done. If you want to make an ISO for future needs, you have to do it before starting the upgrade process itself, after downloading the upgrade files but before clicking Restart now button!