New Post: SSH.NET

What version of SSH.NET are you using ?
Can you verify whether SshClient.IsConnect is still true after the exception ?

Also try subscribing to the Exception event to learn more on the reason why the connection was dropped:

            using (var client = new SftpClient(...))
            {
                client.ErrorOccurred += (sender, args) => Console.WriteLine(args.Exception);
                client.Connect();
                ...
            }