Getting Error when using Workflow with Profil: "User # doesn't exist in UPA by UPN or SID, and user with this SID was not found in AD"

Hello,

Ich have a problem with a SharePoint 2013 Workflow on SharePoint Online.

The Workflow is basically doing a rest call with the following url:

https://sitename.sharepoint.com/_api/SP.UserProfiles.PeopleManager/GetPropertiesFor(accountName=@v)?@v='loginName'

Using this link in the browser returns the correct user as expected

And before that I was following the content of this Link:

https://msdn.microsoft.com/en-us/library/office/jj822159.aspx

It worked because I am not getting any permission denied error.

My Workflow looks like this:

Basically I am just setting the header (application/json;odata=verbose), doing the REST call and sending the Result to my email

The full response I am getting is the following:

{"error":
        {"code": "-1, Microsoft.Office.Server.UserProfiles.UserNotFoundException", "message":
                {"lang": "de-DE", "value": "User 'i:0i.t|00000003-0000-0ff1-ce00-000000000000|app@sharepoint' doesn't exist in UPA by UPN or SID, and user with this SID was not found in AD."
                }
        }
}

Does anybody know what I am doing wrong? I haven't made any mistake in code, because there is none. ;-)

I also found out that the login name "i:0i.t|00000003-0000-0ff1-ce00-000000000000|app@sharepoint" is apperently SharePoint itself.