I have attempted a federation of the ArcGIS Server 10.6 to the ArcGIS Portal. It seemed to have worked as I have also configured the portal to work with our Active Directory domain. However, I cannot register the ArcGIS Server as a 'Hosting Server' because it is not available in the dropdown list. I then noticed the following error when I highlight the server reference in the federated server list:
ArcGIS Server services URL 'https://webadaptorname.domain.local/arcgis' cannot be validated against 'https://webadaptorname.domain.local/arcgis/rest/info'. If the service URL is a proxy URL it is accessible to clients. Error: The server at 'https://gisserver.domain.local:6443/arcgis/admin/info' returned an error. Could not decrypt token. Token may not be valid.
Can you please advise where I may be going wrong?