window service with ssh.net crash session operation has timed out and not running

I created window service that use SSH.net to read file from sFTP server every 600 seconds in OnStart I create timer

timer = new Timer(TimerCallback, null, 0, timeDelay);
and TimerCallBack is function that use SSH.net. In connection before read file I have code to connect to sFTP server like below

        var FILE_READ_FLAG = false;
        try
        {
            req = new SftpClient(ftpHost, ftpLogin, ftpLoginPassword);
            req.Connect();
            if (req.IsConnected)
            {
                directories = req.ListDirectory(ftpPath);
                FILE_READ_FLAG = true;
            }
        }
        catch (Exception e)
        {

            EventLog.WriteEntry(sSource,"ERROR : " + GetFullMessageFromException(e), 
                                EventLogEntryType.Information, 234);

            FILE_READ_FLAG = false;
        }
        if (FILE_READ_FLAG){
           // read file and insert to database
        }

the problem is when service run and exception occur the service can run again every 600 seconds but with the error

Session operation has timed out
   at Renci.SshNet.Session.WaitOnHandle(WaitHandle waitHandle, TimeSpan timeout)
   at Renci.SshNet.Session.WaitOnHandle(WaitHandle waitHandle)
   at Renci.SshNet.Session.Connect()
   at Renci.SshNet.BaseClient.Connect()

the service cannot run again and when I try to stop and restart service, Windows show alert message cannot stop in timely fashion and I have to kill service by cmd and start it manually

My question is why another exception can handle with try/catch block but with this error the service not run again with timer what I missing or using it wrong? thank you

Start with the TimerCallback and the SftpClient.Connect() call described in the report, then trace the Session.WaitOnHandle timeout in the provided stack trace. Reproduce the Windows service behavior around the 600-second timer and determine whether the reported failure is in SSH.NET or the service lifecycle; done requires a clear cause and a reproducible case.