Try changing your web.config identityConfiguration to this:
<issuerNameRegistry type="Webapp1.WsFederationRequestValidator, Webapp1" />
and see if that helps. Also, make sure your reference to System.IdentityModel.Tokens.ValidatingIsserNameRegistry
is correctly setup.
You may have to pull the latest Microsoft Token Validation Extension for Microsoft .Net Framework 4.5
from NuGet.