I've just installed ArcGIS Enterprise using the Enterprise Builder - so this comes with ArcGIS Server, Portal, two webadaptors (server and portal), and Datastore.
I can access Portal just fine using the credentials that I was asked for during the install, however I cannot access the Server Manager. I keep getting an "Unauthorized access" message, as below:

Anyone got any ideas? This is the last part in this (very long) saga I feel, as I just need to get into Server Manager and register my data source.