Excel COM Automation Failure After September 2026 Updates on Office 2021 / Windows Server

Cheng Wang 120 Reputation points
2026-09-09T19:35:51.81+00:00

We appear to have encountered a regression in Excel COM automation after upgrading Microsoft Office 2021 from build 16.0.14334.20848 to 16.0.14334.20906.

Environment

  • Microsoft Office 2021 (Click-to-Run)
  • Windows Server hosted on AWS EC2
  • C# application using Microsoft.Office.Interop.Excel
  • No application code changes between working and failing environments

Working vs. Failing Versions

Office Version (Working) **16.0.14334.20848
**Office version **(**Failing) 16.0.14334.20906
The identical application code continues to work successfully on systems running Office build 16.0.14334.20848.

Problem

After upgrading to Office build 16.0.14334.20906, Excel automation fails during worksheet copy operations.

Error message in .NET:
We couldn't copy this sheet.

  at System.Dynamic.ComRuntimeHelpers.CheckThrowException(Int32 hresult, ExcepInfo& excepInfo, UInt32 argErr, String message)

  at CallSite.Target(Closure , CallSite , ComObject , Object , Object )

  at CallSite.Target(Closure , CallSite , Worksheet , Object , Object )

Error Message in Event Viewer:
Faulting application name: EXCEL.EXE, version: 16.0.14334.20906, time stamp: 0x6a9f74eb Faulting module name: mso20win32client.dll, version: 0.0.0.0, time stamp: 0x6a9f6a0c Exception code: 0x03006542 Fault offset: 0x0000000000236f6d Faulting process id: [Moderator note: Personally Identifiable Information removed] Faulting application start time: 0x01dd407cb96d3187 Faulting application path: C:\Program Files\Microsoft Office\Root\Office16\EXCEL.EXE Faulting module path: C:\Program Files\Common Files\Microsoft Shared\Office16\mso20win32client.dll Report Id: [Moderator note: Personally Identifiable Information removed] Faulting package full name:  Faulting package-relative application ID:

Microsoft 365 and Office | Office Online Server

4 answers

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  1. Cheng Wang 120 Reputation points
    2026-09-10T17:46:07.77+00:00

    After further checking, I can now reproduce the issue directly in Excel rather than from our .NET application. The "Copy-Paste" function in Excel is completely broken in Version 2108 (build 14334.20906). On one of our servers (newer Office 2021 version after the 9/8 patch), if I open Excel, I can no longer copy & paste a cell, whether using "Ctrl-C / Ctrl-V" or the context menu. On another of our server with old build of Office 2021, copy-paste still works. This matches the error message "We couldn't copy this sheet" from our .NET application.

    We have to roll back to the previous version 16.0.14434.20848 using the following command:

    cd "C:\Program Files\Common Files\Microsoft Shared\ClickToRun"

officec2rclient.exe /update user updatetoversion=16.0.14334.20848

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  2. Hendrix 990 Reputation points Independent Advisor
    2026-09-09T20:50:36.5666667+00:00

    Hi Cheng Wang,

    Regarding your concern, over the past few days on the forum, several users have reported experiencing several issues after installing the New Security patch on September 08, 2026 (KB5002914). As of now, there is no official information/article from Microsoft addressing this issue.

    As a temporary solution, I highly recommend uninstalling the newest patch and rolling back to the previous working version. You can follow the guidance in this article to roll back your Excel version How to revert to an earlier version of Office | Microsoft Support

    As a user, I do not have the necessary tools or permissions to resolve it. I recommend that you report the problem in Excel by navigating to Help > Feedback > Report a Problem to help raise visibility of the issue and provide additional diagnostic information to Microsoft.

    I will also monitor this issue and notify you promptly if there are any updates from Microsoft.

    Kindly let me know when there are updates or if you need further assistance. Any updates you’re able to share would be really helpful. I appreciate your time and look forward to hearing how things are going.

    Thank you for your patience and understanding.

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  3. Sean Chen 0 Reputation points
    2026-09-11T04:08:19.2033333+00:00

    Same regression here on Office LTSC 2024 (volume licensed), so it is not limited to Office 2021.

    • Broken: Version 2408 Build 16.0.17932.20976 (September 8, 2026 update)
    • Working: Build 16.0.17932.20910 (August 11, 2026) and 16.0.17932.20162
    • Windows 11 (build 26200), x64

    Symptoms (both in the Excel UI via VBA and via COM automation):

    • Worksheet.Copy fails with "We couldn't copy this sheet." - even on a brand-new blank workbook
    • Range.AutoFill fails ("AutoFill method of Range class failed")
    • Reproduces in a clean automation instance (no add-ins, no XLSTART), so it is not caused by add-ins or the workbook content

    Minimal repro (PowerShell):

    $xl = New-Object -ComObject Excel.Application
$wb = $xl.Workbooks.Add()
$wb.Worksheets.Item(1).Copy()   # -> We couldn't copy this sheet.

    Workaround: rolled back with OfficeC2RClient.exe /update user updatetoversion=16.0.17932.20910 and disabled updates; the same script then passes. For reference, Office 2024 retail 16.0.20326.20144 (released the same day) is not affected on another machine.

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