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0x80240fff

An unexpected Windows Update error, often seen when the update server sends something the PC cannot handle.

How to fix it

1. Restart and try again later: it is often temporary on Microsoft's side.
2. Open Command Prompt as administrator.
3. Stop the update services: net stop wuauserv, then net stop cryptSvc, then net stop bits, then net stop msiserver
4. Rename the update caches: ren C:\Windows\SoftwareDistribution SoftwareDistribution.old and ren C:\Windows\System32\catroot2 catroot2.old
5. Start the services again: net start wuauserv, net start cryptSvc, net start bits, net start msiserver
6. Restart the PC and run Windows Update again.
7. If one update keeps failing, download it by its KB number from the Microsoft Update Catalog (catalog.update.microsoft.com) and install it by double-clicking the file.

Is it serious?

Often clears on its own within a day.