How to unfederate a server which was left federated before uninstalling Portal?

[deleted-user-78LoOqdsdZnJ]

After portal got corrupted on HA, I uninstalled and reinstalled on both the machines. Portal URL was accessible and working fine. When I tried federating a server to this portal it threw a message 'ArcGIS Server is already a federated server'. Due to corruption of portal I could not access Portal admin URL and could not unfederate this server before uninstalling portal. Now, when I want to federate this server with freshly installed Portal it is showing that it is already federated. How to unfederate this server?

Please provide your suggestions/ideas.

[JonathanQuinn]

In the Admin API of the Server, under security > config, click Update and set the Authentication Tier to GIS_SERVER from ARCGIS_PORTAL. That does the same thing that unfederating when all components are accessible would do.

[deleted-user-78LoOqdsdZnJ]

I already did that. It gives the error 'Servlet execution threw an exception'. It gives this error on performing any task in Admin URL. 

[WarrenMedernach]

Hey @JonathanQuinn ,

I am experiencing a similar issue right now after this restore we've completed, and when I attempt to set the Authentication tier in the server admin it errors out with:

Failed to update the security configuration. Error encountered while changing the server role. Server machine 'https://<FQDN>:6443/arcgis/admin/' returned an error. 'Unauthorized access. Token not found. You can generate a token using the 'generateToken' operation.'

Have you seen that before?