c# socket disconnect

Hi,

Ich möchte ein simples chatprogramm schreiben über das zwei clients 
kommunizieren. Ich kann verbindung mit einem Client aufbauen, senden und 
empfangen. Aber wenn ich die Verbidung trenne und mit einer anderen 
Portnummer eine neue Verbindung aufbauen möchte wird eine Exception 
ausgelöst. Exception befindet sich im Anhang! Kann mir jemand 
weiterhelfen? Verbindungstrennung soll in Funktion button3_Click(...) 
stattfinden.

[c]
 Socket sck;
        EndPoint epLocal, epRemot;

        public Form1()
        {
            InitializeComponent();
            sck = new Socket(AddressFamily.InterNetwork, 
SocketType.Dgram, ProtocolType.Udp);
            sck.SetSocketOption(SocketOptionLevel.Socket, 
SocketOptionName.ReuseAddress, true);

            textBoxIPClient1.Text = getLocalIp();
            textBoxIPClient2.Text = getLocalIp();

            button2.Enabled = false;

        }

        private string getLocalIp()
        {
            IPHostEntry host;
            host = Dns.GetHostEntry(Dns.GetHostName());

            foreach (IPAddress ip in host.AddressList)
            {
                if (ip.AddressFamily == AddressFamily.InterNetwork)
                {
                    return ip.ToString();
                }
            }

            return "127.0.0.1";
        }

        private void MessageCallBack(IAsyncResult aResult)
        {
            try
            {
                int size = sck.EndReceiveFrom(aResult, ref epRemot);
                if (size > 0)
                {
                    byte[] receivedData = new byte[1464];
                    receivedData = (byte[])aResult.AsyncState;

                    ASCIIEncoding eEncoding = new ASCIIEncoding();
                    string receivedMessage = 
eEncoding.GetString(receivedData);
                    listBox1.Items.Add("Friend: " + receivedMessage);
                }
                byte[] buffer = new byte[1500];
                sck.BeginReceiveFrom(buffer, 0, buffer.Length, 
SocketFlags.None, ref epRemot, new AsyncCallback(MessageCallBack), 
buffer);
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.ToString());
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {

            try
            {
                epLocal = new 
IPEndPoint(IPAddress.Parse(textBoxIPClient1.Text), 
Convert.ToInt32(textBoxPORTClient1.Text));
                sck.Bind(epLocal);

                epRemot = new 
IPEndPoint(IPAddress.Parse(textBoxIPClient2.Text), 
Convert.ToInt32(textBoxPORTClient2.Text));
                sck.Connect(epRemot);

                byte[] buffer = new byte[1500];
                sck.BeginReceiveFrom(buffer, 0, buffer.Length, 
SocketFlags.None, ref epRemot, new AsyncCallback(MessageCallBack), 
buffer);

                button1.Text = "Connected";
                // button1.Enabled = false;

                button2.Enabled = true;
                textBoxMessage.Focus();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.ToString());
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            try {
                System.Text.ASCIIEncoding enc = new 
System.Text.ASCIIEncoding();
                byte[] msg = new byte[1500];
                msg = enc.GetBytes(textBoxMessage.Text);
                sck.Send(msg);

                listBox1.Items.Add("You: " + textBoxMessage.Text);
                textBoxMessage.Clear();

            }
            catch(Exception ex){
                MessageBox.Show(ex.ToString());
            }
        }

        private void button3_Click(object sender, EventArgs e)
        {
            try
            {
                sck.Disconnect(true);
                sck.Close();

            }catch(Exception ex){
                MessageBox.Show(ex.ToString());
            }
        }

[\c]