Potential problems and causes
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Workaround is to have users deactivate the Pop-Up-Blocker at least for their confluence domain as originating (and https://login.microsoftonline.com/ as domain of the pop-up if this can/has to be specified). Other users might also have the problem when not using the suport tool but inserting or viewing a SharePoint Online List/Document macro - they have to "login" at least once to connect our application to SharePoint Online, even if they are already logged in into SharePoint Online or Office 365.
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Permissions have to be configured and after that consent has to be granted by an admin for application in Azure portal. Might take a view few minutes to update.
Either timeout or blocked third party cookies; to narrow down the issue use the button.
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