When using this code I get the above error:
        var userSettings = new UserSettings()
        {
            Username = TextBox_Username.ToString(),
            Password = TextBox_Password.ToString(),
            Operator = OperatorList.O2
        };
        var settings = IsolatedStorageSettings.ApplicationSettings;
        settings.Add("UserSettings", userSettings);
        settings.Save();
I'm not sure why? though.
Fixed it. Any stored class needs to be marked as public marking classes internal is not allowed. The issue happened because C# does not default to public scope with new classes.
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