Error message when starting a Microsoft Office third-party add-in that relies on .NET binaries

Symptoms

You receive one of the following kinds of error messages when attempting to start a third-party Office add-in that relies on .NET framework Office binaries:

  • "Access Denied"
  • "File not found or no read permission"
  • "Could not load file or assembly‚Ķ"

Cause

Devices affected by this issue were sold with Windows 10 RS5, release 17763, which came bundled with Centennial Office, a version of Office once available through the Windows Store. Even if Centennial Office has never been activated, Windows Store continues to update the associated installation packages on the affected device, a process which changes and severely restricts permissions on ".DLLs" located in subfolders of the Office Primary Interop Assembly (PIA) library folder, C:\Windows\assembly\GAC_MSIL.

The restricted permissions on the PIA files prevent many third-party add-ins from operating. The most common add-ins affected are those for Excel, which often use files in the folder:

C:\Windows\assembly\GAC_MSIL\Policy.14.0.Microsoft.Office.Interop.Excel\15.0.0.0__71e9bce111e9429c 

If the Centennial Office package files are not removed from the affected computer, the issue will reoccur when Centennial Office receives an update from the Microsoft Store.

Resolution

Users should run the Online Repair tool on the affected device. The Online Repair Tool will remove the Centennial Office packages and reestablish the permissions on the affected PIA files.

More information

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