"We can't open this add-in from localhost" when loading an Office Add-in or using Fiddler


Office 365 ProPlus is being renamed to Microsoft 365 Apps for enterprise. For more information about this change, read this blog post.


When you load an Office Add-in from https://localhost or when you use Fiddler, you receive the following error:

We can't open this add-in from localhost.


One cause of this error is that the Microsoft Edge Web Viewer does not have a loopback exemption. To resolve this issue, use either of these options to add a local loopback exemption to Desktop App Web Viewer:

Option 1: Use command to add a local loopback exemption

  1. Open a command prompt as administrator. 
  2. Run the following command:
    CheckNetIsolation LoopbackExempt -a -n="microsoft.win32webviewhost_cw5n1h2txyewy" 

Option 2: Use Fiddler to add a local loopback exemption

  1. Select Tools > Win8 Loopback Exemptions.
  2. Add an exemption to Desktop App Web Viewer.

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