Hi Dyari,
I have never used another account on this machine, either work or school related.
Also, there are no files protected by another person / organisation, they have all been created by myself and not been transferred from another source.
When I googled the issue, it says ' The classic Azure Information Protection (AIP) / Purview viewer has strict rules regarding personal versus organizational credentials, and certain protected file types do not support personal Microsoft accounts natively in the viewer app'.
I assume there should be a way to unlock this and allow the Information Protection Viewer to recognise the original account.