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

Note

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

Symptoms

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.

Resolution

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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