I log in to my administrator account on Portal for ArcGIS
click on My Organization->Edit Settings->Servers
In the servers list there is a server which was federated with the Portal previously.
At present when I want to Remove server from the Portal, it throws me an error saying
"Unable to update the authentication tier of the server prior to unregistering it from the portal. The server has not been changed or unregistered."
That server never gets removed and even when I try to over write it by adding it as a new server it shows This server is already registered in ArcGIS Portal.
I even changed the Authentication tier as ArcGIS Server at Security->Config
Is there is a solution for this?