I wanted to know if anyone else had this experience and to see if you found a solution. I was in the process of upgrading my ArcGIS Enterprise to 10.7.1 from 10.5.1 and was following the steps. All the upgrading processes were performing as expected until I encountered the ArcGIS Server section. Initially, it appeared that the upgrade was successful but tried accessing the service directory on my local machine and was unsuccessful.
I also knew that I was encountering an error when I attempted to register the web adapter with the server and I received an "ArcGIS web adaptor server failed to get administer token from the server. Please verify that the ArcGIS Server URL specific can be accessed successfully" error.
I just logged a support ticket but seeing if the community ran into this issue.