Hello,
I did a fresh ArcGIS Portal 10.9.1 installation (on my laptop and on a server).
Site creation is successfully completed and I can login to arcgis/home with portal admin user.
But I am not able to log in portaladmin.
When I go to "https://portal.host.machine:7443/arcgis/portaladmin/" I see below screen.

I have tried on different browser (Edge, Firefox, Chrome, IE), same result.
I have no error in the portal logs.
Do you have any idea to solve the problem?
Thanks