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Detailed explanations can be found at https://docs.microsoft.com/fr-fr/office/dev/add-ins/testing/troubleshoot-manifest
Enabling add-in on office online
In order to enable the add-in in the online version of Office, you need to allow iframe integration for the entry point.
This can be done by creating a specific rule in the admin module “HTTP safety headers”.
fill-in acceptRelativeUrls with the URL pointing to the picker entry point (should be /office-picker/office-asset-picker.html)
For this rule, you should diable (uncheck) XFramed option