Some users of Windows 7 or Windows 8.1 are receiving error 80240020 from Windows Update during a Windows 10 upgrade. And the upgrade stalls.
The reason for this error is, that the 1st install phase can't be finished. In German Microsoft Answers forum our moderators suggest to wait till the system notifies about the Windows 10 upgrade. Another recommendation is to proceed the following steps.
1. Open an administrative command prompt window and execute the command net stop wuauserv – this stops Windows Update service.
2. Open explorer, navigate to C:\Windows\SoftwareDistribution\Download\ and delete all files included within this folder.
3. Execute the command wuauclt.exe /updatenow within an administrative command prompt window.
After rebooting the error should be gone. For further reading, visit this reddit.com thread.
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