Ho risolto il problema aggiungendo 
	codice:
	if(!Page.IsPostBack)
 
nel blocco Load.
	codice:
	namespace MyApp.UserHub
{
    public partial class EditUserAccount : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if(!Page.IsPostBack)
            { 
            if(Session["ActiveUserId"] == null)
            {
                Response.Redirect("../Login.aspx");
            }
            else
            {
                string connString = ConfigurationManager.ConnectionStrings["conn"].ConnectionString;
                string ActiveUserId = Session["ActiveUserId"].ToString();
                    using (SqlConnection conn = new SqlConnection(connString))
                    {
                        using (SqlCommand cmd = new SqlCommand("SELECT * FROM Utente WHERE UserId=@ActiveUserId ", conn))
                        {
                            cmd.Parameters.AddWithValue("@ActiveUserId", ActiveUserId);
                            conn.Open();
                            using (SqlDataReader result = cmd.ExecuteReader())
                            {
                                while (result.Read())
                                {
                                    txtID.Text = HttpContext.Current.User.Identity.Name;
                                    txtName.Text = result["Name"].ToString();
                                    txtSurname.Text = result["Surname"].ToString();
                                    lblRegistrationDate.Text = result["InsertDate"].ToString();
                                }
                            }
                        }
                    }
                }
            }
               
        }
        protected void EditButton_Click(object sender, EventArgs e)
        {
     
                string connString = ConfigurationManager.ConnectionStrings["conn"].ConnectionString;
                string ActiveUserId = Session["ActiveUserId"].ToString();
                string u_Name = txtName.Text.ToString();
                using (SqlConnection conn = new SqlConnection(connString))
                {
                    using (SqlCommand cmd = new SqlCommand("UPDATE Utente SET Name= @Name  WHERE UserId= @UserId", conn))
                    {
                        cmd.Parameters.AddWithValue("@Name", u_Name);
                        cmd.Parameters.AddWithValue("@UserId", ActiveUserId);
                        conn.Open();
                        cmd.ExecuteNonQuery();
                        // Response.Write("Occorrenze trovate:" + edited.ToString());
                    }
                }
            
        }
    }
}