C# 2nd call to InternetExplorer ie in new ShellWindows() fails

I am using the following C# code to automatically populate and validate a web page:

namespace Web_Browser_Manipulation

{
    public partial class Form1 : Form
    {
    AutoResetEvent waitHandle = new AutoResetEvent(false);

    public Form1()
    {
        InitializeComponent();
    }

    public void ie_DocumentComplete(object pDisp, ref object URL)
    {
        InternetExplorer ie = pDisp as InternetExplorer;
        ie.DocumentComplete -= ie_DocumentComplete;
        waitHandle.Set();
    }

    private void UploadSignature(string Nome_SW, string Codice_SW, string Codice_Conc, string Codice_Gioco)
    {
        foreach (InternetExplorer ie in new ShellWindows())
        {
            if (ie.LocationURL.Contains("aams"))
            {
                int index = FindLineIndex(Codice_Conc, Codice_Gioco);
                ie.DocumentComplete += ie_DocumentComplete;

                if (index != -1)
                {
                    ie.Document.GetElementsByTagName("option")[index].SetAttribute("selected", "true");
                    ie.Document.GetElementsByTagName("input")["aggiungi"].Click();

                    ie.Document.GetElementById("num").SetAttribute("value", "1");
                    ie.Document.GetElementsByTagName("input")["prepara"].Click();
                    ie.Document.GetElementById("nome_sftw1").SetAttribute("value", Nome_SW);
                    ie.Document.GetElementById("cod_sha11").SetAttribute("value", Codice_SW);
                    ie.Document.GetElementsByTagName("input")["Acquisisci"].Click();
                }
            }
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        UploadSignature("Nome1", "1234567890123456789012345678901234567890", "15023", "1");
        waitHandle.WaitOne();
        UploadSignature("Nome2", "1234567890123456789012345678901234567891", "15023", "1");
        waitHandle.WaitOne();
    }
}

The code works great, however, when the function "UploadSignature" is called the 2nd time, the "InternetExplorer ie" fails and therefore the program crashes (it raises an exception:'(((SHDocVw.InternetExplorer)(ie)).Document).baseUrl' generated 'System.NotImplementedException' and fileCreatedDate = '(((SHDocVw.InternetExplorer)(ie)).Document).fileCreatedDate' generaed 'System.Runtime.InteropServices.COMException'). When the program reaches the line

ie.Document.GetElementsByTagName("option")[index].SetAttribute("selected","true");

it crashes.

I really do not mange to figure out why. Could anyone help? Thank you so much for any hint you might provide!