Issue summary
The username is not working when trying to login into Windows, Remote Desktop or any other service including Office Portal, in the sample case below it is exactly the same issue, the user Jan.Venter did not want to add to the access list for reporting and when checked online in Azure portal, the user exists and no issues visible.
The username below is different, it was just a sample to show that the user was not seen by die server to add.
The issue that I have found multiple times before is that the "Mail nickname" - It should always be the user's username e.g. jan.venter in his it was filled in as "tracking", to fix this click on "Contact Information" on the pencil and update the Mail nickname.
The issues would then be resolved and the login for tracking will not work anymore.