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

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


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?