Windows Update 0x800700C1 Error

Hi there

Currently running 24H2 v26100.7171

trying to install the December 2025 update (2025-12 Security Update (KB5072033) (26100.7462)) and receiving 0x800700C1
after many attempts to fix, I then attempted to install 25H2 which I've had optionally sat there a little while (Windows 11, version 25H2) - again this fails with 0x800700C1

I've tried running Brinks WUReset - as found here - ARTICLE - Reset Windows update components - Microsoft Q&A

I've ran SFC /ScanNow (successfully, no errors)
I've ran DISM /Online /Cleanup-Image /RestoreHealth (successfully, no errors)
I've ran the same DISM RestoreHealth but used another machine on the domain which successfully has updated (also successful, wihtout errors)
I've ran CHKDSK C: /F /R /X all ok
I've ran the Windows Update Troubleshooter - it did its thing, but made no difference

Does anyone else have any ideas what I can try please?

The errors I'm receiving from the WindowsUpdate.log file are:

2025/12/16 10:20:29.6667250 4524 8328 Handler *FAILED* [800700C1] UUP GDR: Completed
2025/12/16 10:20:29.6667382 4524 8328 Handler *FAILED* [800700C1] wuuhosdeployment.dll, C:\__w\1\s\src\Client\Engine\handler\OSDeployment\Downloader\OSAsyncDownloader.cpp @213
2025/12/16 10:20:29.6731721 4524 8328 Handler *FAILED* [800700C1] wuuhosdeployment.dll, C:\__w\1\s\src\Client\Engine\handler\OSDeployment\Handler\UHOSDeployment.cpp @345
2025/12/16 10:20:29.6731815 4524 8328 Agent *FAILED* [800700C1] wuauengcore.dll, C:\__w\1\s\src\Client\Engine\Agent\Download\GDRHelper.cpp @376
2025/12/16 10:20:29.6731833 4524 8328 DownloadManager *FAILED* [800700C1] GDR
2025/12/16 10:20:30.0204056 4524 8324 DownloadManager Reporting DownloadQOS event for update 8F24A30B-B9F9-4608-9E79-2947E26360EC.1. Props: 0xd0, HandlerId: OSDeployment, DownloadStartTimeUTC: 1601-01-01 00:00:00, DownloadPriority: 2, ClassificationId: 3689BDC8-B205-4AF4-8D4A-A63924C5E9D5, BundleId: 00000000-0000-0000-0000-000000000000.0, FlightId: RS:2EAE8, NetworkCost: 1, NetworkRestrictionStatus: Unrestricted, HostName: tlu.dl.delivery.mp.microsoft.com, DownloadResult: DownloadFailed, MaxGDRTimeInMs: 704395
2025/12/16 10:20:30.0268364 4524 8324 DownloadManager Uninit download job for update 8F24A30B-B9F9-4608-9E79-2947E26360EC.1, downloadType: 0.
2025/12/16 10:20:30.0274152 4524 4676 Agent Reschedule the tasks in general work item if they are waiting to execute.
2025/12/16 10:20:30.0274168 4524 8328 Handler OnDownloadJobFailure - UUP update 8F24A30B-B9F9-4608-9E79-2947E26360EC.1 in the sandbox C:\WINDOWS\SoftwareDistribution\Download\a367f1d3f4e821e3708a35c74f81383f failed with 0x800700c1.
2025/12/16 10:20:30.0274309 4524 8328 Handler OnDownloadJobFailure - UUP update 8F24A30B-B9F9-4608-9E79-2947E26360EC.1 found 0l downloaders.
2025/12/16 10:20:30.0367025 4524 8328 DownloadManager *FAILED* [800700C1] Error occurred while downloading update 8F24A30B-B9F9-4608-9E79-2947E26360EC.1; notifying dependent calls.
2025/12/16 10:20:30.0373709 4524 4676 Agent Reschedule the tasks in callback work item if they are waiting to execute.
2025/12/16 10:20:30.0373720 4524 4676 Agent Unhandled work item type: callback work item.
2025/12/16 10:20:30.1608647 4524 8328 Agent *FAILED* [800700C1] wuauengcore.dll, C:\__w\1\s\src\Client\Engine\Agent\Download\GDRHelper.cpp @45
2025/12/16 10:20:30.1608689 4524 8328 DownloadManager Unlock the sandbox of update 8F24A30B-B9F9-4608-9E79-2947E26360EC.1 (session data: (null))
2025/12/16 10:20:30.1658360 4524 8328 Agent *FAILED* [800700C1] wuauengcore.dll, C:\__w\1\s\src\Client\Engine\Agent\Download\GDRHelper.cpp @11
2025/12/16 10:20:30.1658373 4524 8328 Agent *FAILED* [800700C1] wuauengcore.dll, C:\__w\1\s\src\Client\Engine\Agent\downloadmanager.cpp @3903

Any help would be very much appreciated please!

Excerpt of the original log attached for easier formatting than what I pasted in the initial post
Also, attached is the error message received when trying to run the update using the standalone installed, having downloaded from MS
"%1 is not a valid Win32 application."

You can add this to what you have already done..
CLEAN UP COMPONENT STORE - dism /online /cleanup-image /startcomponentcleanup
Analyze and Clean Up Component Store (WinSxS folder) in Windows 11 Tutorial

Try a repair install. Repair Install Windows 11 with an In-place Upgrade Tutorial

Thanks glasskuter, the /startcomponentcleanup is one of my usual post-windows-update monthly commands I run, so I had tried that earlier on (though forgot to list on my OP) - it originally errored, but I didn't screenshot at the time as I was unaware of the larger issue

As we speak, I am running a repair install (through Windows Update) - fingers crossed that does the job

SV IT Support said:

I am running a repair install (through Windows Update)

Click to expand...

I hope so too. I always use the iso myself.

Good news - the repair install did the trick (and as part of it, updated from 26100.7171 to 26100.7462 - the original issue)
I'll keep that one in my back pocket for future use for sure.