Обновлен Visual Studio до 15.8.5 и библиотека AWS AWSSDK.Extensions.CognitoAuthentication перестала работать с ошибкой выше
Amazon.AWSConfigs.AWSRegion = AWSConfigConstants.Region;
var provider = new AmazonCognitoIdentityProviderClient(new AnonymousAWSCredentials(), FallbackRegionFactory.GetRegionEndpoint());
var userPool = new CognitoUserPool(AWSConfigConstants.PoolId, AWSConfigConstants.ClientId, provider);
var user = new CognitoUser(model.UserName, AWSConfigConstants.ClientId, userPool, provider);
authResponse = await user.StartWithSrpAuthAsync(new InitiateSrpAuthRequest()
{
Password = model.Password
}).ConfigureAwait(false);