Ошибка при работе с XML - C#

<?xml version="1.0" encoding="UTF-8"?>
<User xmlns="urn:task-manager">      
        <users>
            <Login>Superuser</Login>
            <FirstName>Максим</FirstName>
            <LastName>Аверкин</LastName>
            <Locale>ru</Locale>
            <TimeZone>E. Europe Standard Time</TimeZone>
            <RegistrationDate>2014-10-03T00:00:00</RegistrationDate>
            <FullName>Максим Аверкин</FullName>
            <Password>5f4dcc3b5aa765d61d8327deb882cf99</Password>
            <Role>User</Role>
            <Id>0d2da234-6b23-41db-b25a-3310add1d89a</Id>
        </users>
        <users>
            <Login>LittlePonny</Login>
            <FirstName>Ольга</FirstName>
            <LastName>Нефедова</LastName>
            <Locale>ru</Locale>
            <TimeZone>E. Europe Standard Time</TimeZone>
            <RegistrationDate>2014-11-06T00:00:00</RegistrationDate>
            <FullName>Ольга Нефедова</FullName>
            <Password>25f9e794323b453885f5181f1b624d0b</Password>
            <Role>User</Role>
            <Id>c2b0bd56-65f9-4c26-a972-a17e52b7a461</Id>
        </users>
        <users>
            <Login>Smuzy</Login>
            <FirstName>Олег</FirstName>
            <LastName>Смойлов</LastName>
            <Locale>ru</Locale>
            <TimeZone>E. Europe Standard Time</TimeZone>
            <RegistrationDate>2014-05-10T00:00:00</RegistrationDate>
            <FullName>Олег Смойлов</FullName>
            <Password>ee95a16d763ab0d26ee62c53056df928</Password>
            <Role>User</Role>
            <Id>3b411036-af57-413c-812c-6bee5c39ddac</Id>
        </users>
        <users>
            <Login>Ragana</Login>
            <FirstName>Кристина</FirstName>
            <LastName>Вершинина</LastName>
            <Locale>ru</Locale>
            <TimeZone>E. Europe Standard Time</TimeZone>
            <RegistrationDate>2015-10-01T00:00:00</RegistrationDate>
            <FullName>Кристина Вершинина</FullName>
            <Password>673eb027e9c056f57140322807351dd5</Password>
            <Role>User</Role>
            <Id>0244be75-fc0e-456e-9471-2846ed05736d</Id>
        </users>
        <users>
            <Login>Akakiy</Login>
            <FirstName>Игорь</FirstName>
            <LastName>Кривицкий</LastName>
            <Locale>ru</Locale>
            <TimeZone>E. Europe Standard Time</TimeZone>
            <RegistrationDate>2015-02-20T00:00:00</RegistrationDate>
            <FullName>Игорь Кривицкий</FullName>
            <Password>cb1d3a6249c2d223c620393fa6420868</Password>
            <Role>User</Role>
            <Id>dd073703-68ef-4b3a-bd70-d4a19a87e8e1</Id>
        </users>
        <users>
            <Login>Lary</Login>
            <FirstName>Сергей</FirstName>
            <LastName>Микулич</LastName>
            <Locale>ru</Locale>
            <TimeZone>E. Europe Standard Time</TimeZone>
            <RegistrationDate>2015-01-03T00:00:00</RegistrationDate>
            <FullName>Сергей Микулич</FullName>
            <Password>6ab3d0526bb1d023ab7c4fee4fb684df</Password>
            <Role>User</Role>
            <Id>10f387d8-3671-40dd-a4f8-999ff3975ddb</Id>
        </users>
        <users>
            <Login>Grazyy</Login>
            <FirstName>Виталий</FirstName>
            <LastName>Евтухов</LastName>
            <Locale>ru</Locale>
            <TimeZone>E. Europe Standard Time</TimeZone>
            <RegistrationDate>2015-04-24T00:00:00</RegistrationDate>
            <FullName>Виталий Евтухов</FullName>
            <Password>a77224c90dc6867f603a3afd92767598</Password>
            <Role>User</Role>
            <Id>07eb366f-3918-4e52-8ebb-2407a1f96390</Id>
        </users>
        <users>
            <Login>Bigboss</Login>
            <FirstName>Валерий</FirstName>
            <LastName>Маковецкий</LastName>
            <Locale>ru</Locale>
            <TimeZone>E. Europe Standard Time</TimeZone>
            <RegistrationDate>2014-03-08T00:00:00</RegistrationDate>
            <FullName>Валерий Маковецкий</FullName>
            <Password>d8578edf8458ce06fbc5bb76a58c5ca4</Password>
            <Role>User</Role>
            <Id>51c2844b-938c-4ba1-b50a-af6dea8a765d</Id>
        </users>
        <users>
            <Login>Administrator</Login>
            <FirstName>Константин</FirstName>
            <LastName>Горбачёв</LastName>
            <Locale>ru</Locale>
            <TimeZone>E. Europe Standard Time</TimeZone>
            <RegistrationDate>2014-01-02T00:00:00</RegistrationDate>
            <FullName>Константин Горбачёв</FullName>
            <Password>21232f297a57a5a743894a0e4a801fc3</Password>
            <Role>Admin</Role>
            <Id>5fca13d8-91d6-4023-b60a-d9334ec5c983</Id>
        </users>
        <users>
            <Login>Login</Login>
            <FirstName>FirstName</FirstName>
            <LastName>LastName</LastName>
            <Locale>ru</Locale>
            <TimeZone>Paraguay Standard Time</TimeZone>
            <RegistrationDate>2015-05-19T14:35:33</RegistrationDate>
            <FullName>FirstName LastName</FullName>
            <Password>50f3fca4c6134bd001fdfe3159686be9</Password>
            <Role>User</Role>
            <Id>36fb1c67-7363-4cae-a8f9-7b0a9bcdfb3e</Id>
        </users>
        <users>
            <Login>Vinny</Login>
            <FirstName>Владимир</FirstName>
            <LastName>Некрасов</LastName>
            <Locale>en</Locale>
            <TimeZone>E. Europe Standard Time</TimeZone>
            <RegistrationDate>2014-12-15T00:00:00</RegistrationDate>
            <FullName>Владимир Некрасов</FullName>
            <Password>1897a69ef451f0991bb85c6e7c35aa31</Password>
            <Role>User</Role>
            <Id>0ee509d6-8c7d-4070-98dd-9003cb266496</Id>
        </users>
</User>

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using TaskManager.DAL;
 
namespace TaskManager.DAL.Entities
{
    [Serializable]
    public class User : IEntity
    {
        public string Login { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Locale { get; set; }
        public string TimeZone { get; set; }
        public DateTime RegistrationDate { get; set; }
       
        //public string FullName
        //{ 
        //    get { return string.Format("{0} {1}", FirstName, LastName); }
        //}
        public string Password { get; set; }
        public string Role { get; set; }
    }
}

public List<T> GetAll<T>(string path, string pathxsd)
        {
            logger.Trace("Sample trace message");
            logger.Debug("Sample debug message");
            logger.Info("Sample informational message");
            logger.Warn("Sample warning message");
            logger.Error("Sample error message");
            logger.Fatal("Sample fatal error message");
         //   bool result = DtdValidation(path);
            bool resultxsd = XsdValidation(path, pathxsd);
            XmlSerializer serializer = new XmlSerializer(typeof(T));
            List<T> list = new List<T>();
            object locker = new object();
            if (resultxsd == true)

            {
               
                   using (FileStream stream = File.OpenRead(path))
                    {
                        list = (List<T>) serializer.Deserialize(stream);
                   }
                
            }
            return list;
        }

list = (List<T>) serializer.Deserialize(stream);