WSACancelBlockingCall exception

Is it possible that the serverSocket is being closed from another thread? That will cause this exception.

Same here! But i figured out, that the ReceiveBuffer on 'server-side' was flooded from the clients! (In my case a bunch of RFID-Scanners, who kept spamming the TagCode, instead of stop sending until next TagCode arrives)

It helped to raise the ReceiveBuffers and reconfigure the scanners...

This is my example solution to avoid WSAcancelblablabla: Define your thread as global then you can use invoke method like this:

private void closinginvoker(string dummy)
    {
        if (InvokeRequired)
        {
            this.Invoke(new Action<string>(closinginvoker), new object[] { dummy });
            return;
        }
        t_listen.Abort();
        client_flag = true;
        c_idle.Close();
        listener1.Stop();
    }

After you invoke it, close the thread first then the forever loop flag so it block further waiting (if you have it), then close tcpclient then stop the listener.

This could happen on a serverSocket.Stop(). Which I called whenever Dispose was called.

Here is how my exception handling for the listen thread looked like:

try
{
    //...
}
catch (SocketException socketEx)
{    
    if (_disposed)
        ar.SetAsCompleted(null, false); //exception because listener stopped (disposed), ignore exception
    else
        ar.SetAsCompleted(socketEx, false);
}

Now what happened was, every so often the exception would occur before _disposed was set to true. So the solution for me was to make everything thread safe.