Windows 10 V1909: Installation of update KB5003635 fails

Windows[German]Today another small blog post about installation problems with Windows 10 updates with a surprising cause. I came across a case where the installation of update KB5003635 fails on Windows 10 version 1909 clients. The cause is bizarre: an outdated version of Robocopy. Currently, the only option is to upgrade to a newer Windows 10 version or suspend the update.


Update KB5003635 for Windows 10 V1909

Cumulative Update KB5003635 has been available since June 8, 2021 for Windows 10 version 1909 as well as for Windows Server version 1909. It fixes some vulnerabilities and bugs in the Windows 10 build in question. I had reported about this update in the blog post Patchday: Windows 10 updates (June 8, 2021).

Update KB5003635 fails

While researching another bug, I then came across this forum thread on Microsoft's Q&A pages. There an affected person describes the problem:

Unable to install KB5003635 in WSUS

In my testing, my test 1909 machines are seeing all updates except for these, even after reboots:
Windows 10 June 2021 Cumulative Rollup Update for Version 1909 (KB5003635)
June 2021 Servicing Stack Update for Windows 10 Version 1909 (KB5003710)

Both of these patches are listed in WSUS and deployed with everything else, the Office security updates and the Third Party updates from PatchMyPC. All other updates installed just fine.

These computers have the April 13, 2021 servicing stack update (SSU) (KB5001406) and also have the May 11, 2021 Servicing Stack Update for Windows 10 Version 1909 (KB5003244).
I have been deploying Windows Updates from Microsoft via WSUS for 11 years. I worked for Microsoft Technical Support from 1998 to 2004. There have been many Windows 10 versions that have come and gone. All versions that went end of life still had updates available for several months after the technical end of life date. Never yet, until now, have I seen Windows Updates fail to install when they were offered via WSUS.

The administrator fails to have the mentioned update KB5003635 installed on the clients via WSUS. In the description, he mentions that he has already manually installed the required servicing stack updates (SSUs). Reboots did not help either. The update won't install, and Windows 10 version 1909 rolls back. He also tried it on another test machine with the same result. Note here that the whole thing has nothing to do with WSUS – there are others affected without WSUS.

Old Robocopy version is the cause

In the thread BahenaRene-5074 reported that he also observed the problem and is working with Microsoft staff to analyze and solve the problem:

Hey, I am working with a MS Engineer on this right now and one thing we noticed was the machines this is failing on has a super old copy of robocopy from 2003 in C:\Windows\System32\Robocopy.exe. Are you seeing the same thing?

It has been noticed that the affected machines have an old version of robocopy installed in the specified path. The problem was that Microsoft released an update for robocopy.exe, but on the problematic systems a very old version of robocopy was present. Since the hash values did not match, this caused the update to fail.


BahenaRene-5074 solved it by copying robocopy files from a working machine to the troublesome target systems. Microsoft's TeemoTang-MSFT gave the simple recommendation:

  • Since the problem occurred only on 1909 machines, please upgrade your clients to 20H2 or 21H1 for patching if it is feasible.
  • If it is not feasible, I suggest pausing the installation of KB5003635 for a period of time or skipping this KB and waiting for the next one.

The problem is likely to hit more clients in my estimation, so pausing the update is the only solution if you can't upgrade. This case has been submitted to Microsoft and should be fixed in one of the upcoming updates.

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2 Responses to Windows 10 V1909: Installation of update KB5003635 fails

  1. Yanta says:

    That's bizarre. After reading this I checked my Robocopy.exe. I'm on 1809 LTSC. Mine is dated 28/05/2021. I haven't installed the 2021-05 CU yet, my latest CU is April 2021. The April CU was installed on the 10th Of June. I'm not having any problems – just thought it was interesting

  2. Victor says:

    The update fails on my workstation and my Robocopy.exe version is as recent as march 2019

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