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

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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