Change the authenticationmethod claim to be an absolute uri
This one was just plain silliness on my part. I was overriding the login/authentication mechanism for Identity Server. One of the arguments was the Authentication.Method.
Early in my experiments, I set it to an invalid setting and it didn’t seem to have any results/effect. However, when I was wrapping-up my work, I started getting this error (above). The error didn’t happen ANYWHERE near my whacky code change. So it took a while to back-track and find my mistake.
Solution: When calling SignIn() or SetSessionToken() or CreatePrincipal(), make sure that you specify an AuthenticationMethod that is part of the enum System.IdentityModel.AuthenticationMethods.