با عرض سلام

من در asp.net web form برای فرم ورود به مشکل برخورد کرده ام. هر کدی رو که وارد می کنم موقع اجرا با خطای زیر مواجه می شوم :

Invalid postback or callback argument.  Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page.  For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them.  If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.

کد های من این ها هستند که از همین سایت و یا سایت های دیگه پیداشون کردم :

using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;

public partial class content_btn1 : System.Web.UI.Page
{
    protected void btnlogin_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["myconectionstring"].ConnectionString);
        con.Open();
        SqlCommand cmd = new SqlCommand("select * from users_table where Name =@username and Password=@password", con);
        cmd.Parameters.AddWithValue("@username", txtuser.Text);
        cmd.Parameters.AddWithValue("@password", txtpassword.Text);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataTable dt = new DataTable();

        da.Fill(dt);
        if (dt.Rows.Count > 0)
        {  
            Response.Redirect("btn2.aspx");
        }
        else
        {
            lbllogindetails.Text = "Invalid User or Password !!";
        }
    }
}

ویا :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.OleDb;

namespace ProjectDemo_Asp.et
{
    public partial class SimpleLogin : System.Web.UI.Page
    {
        public string connectionstring = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\\bookstore.mdb;Persist Security Info=False;";
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void btnlogin_Click(object sender, EventArgs e)
        {
            DataTable objdt = new DataTable();
            objdt = ValidateUserDetail(txtuserid.Text, txtpassword.Text);
            if (objdt.Rows.Count > 0)
            {
                /*Invoke session value to mantain usee login to other poages
                 For user authentication in secure page
                 */
                Session["userlogin"] = txtuserid.Text;
                Response.Redirect("Success.aspx");
            }
            else
            {
                Page.ClientScript.RegisterStartupScript(this.GetType(), "Correct", "alert('Wrong user id aand password.')", true);
            }
        }
        /// <summary>
        /// Function for binding retribing the data from database
        /// </summary>
        /// <returns></returns>
        public DataTable ValidateUserDetail(string userid, string password)
        {
            DataTable _objdt = new DataTable();
            string querystring = "select * from UserDetail where userid='" + userid + "' and pwd='" + password + "';";
            OleDbConnection _objcon = new OleDbConnection(connectionstring);
            OleDbDataAdapter _objda = new OleDbDataAdapter(querystring, _objcon);
            _objcon.Open();
            _objda.Fill(_objdt);
            return _objdt;
        }
    }
}

آیا کد های من غلط هستند ؟ مشکل از کجا می باشد ؟

با تشکر