Adding a sharepoint extension is failing due 403 (Ensure Extension Pack is valid and does not already exist)

EduardoLopez

I'm trying to add one of the samples sharepoint extensions into my ArcGis map. When I try to do that it returns the following error:

Failed to add Extension Pack from the specified address. Ensure Extension Pack is valid and does not already exist.

Samples—ArcGIS Maps for SharePoint | ArcGIS 

When I check the browser's console I see the following error:

ERROR: 403

URL: https://la.arcgis.com/proxy?[FILEURL]/manifest.json.txt

I made the APP, API and everything as open as possible to find out where is the block, even anonymous usage. No success so far. 

- I can confirm that I can access the manifest.json.txt through the URL

- Arcgis app is registered and I can see redirect URLs from my sharepoint
- ArcGis is authenticated successfully thought the App configuration button

- ArcGis map is working with the default configuration connected to a sharepoint list

- There are plenty of credits available in the organization account

What can I do to solve the issue and add a custom extension?

Why this error is happening?

NickFosbery

I've seen something similar when trying to do this on SharePoint Online where https://*.arcgis.com needed to be added to the list of trusted sites on the machine you're using and ensure the checkbox 'Require server verification (https:) for all site in this zone'  is ticked on.

Update - actually I think this is something larger as I've now got the same problem adding an extension. It was working fine using Chrome and Edge a month or two ago but is now failing with the same error. It does work in IE11 which makes me think that it is some recent Windows or Edge update that's knocked it out.