No, that is definitely not the case. We just have one gateway, and all our machines have one the same public IP address.

Maybe your local machine is connected to two networks? Each having different DNS?

Thanks for the help. This is really interesting. I have never used direct IP for this client (I'm trying to not to use direct IP at all, only if client requires it and insist) DNS name for the host only and did not pay attention to IP addresses. Is it really possible that 1 DNS name could point to 2 different physical IP at the same time, and both IPs are working and belong to one 1 single service? I checked them both using direct IP, and both are the same SFTP server, I can use either one. What was happening is that our service opened 2 connections to the same client within short period of time (not one right after another, but short enough around 40-50 seconds) and if these 2 connections were using different IP addresses that error occurred because FTP server did not allow to open another session for the same user within this short time but did not reject connection request. It was simply waiting and then timeout error occurred. It happened only if second session had different IP. If IP for the 2 sessions are the same everything works as expected. Once I used direct IP instead of DNS host name, I stopped getting errors, no matter which of those 2 IPs I'm using.

Your service connects to a different IP than the GUI. Is that expected? Can you connect to the IP using plain C# code from your service?

Here is the log from WinSCP.exe (GUI). I hope you have all the information now.
Thanks, Boris

Yes, I meant GUI.

Ok, I assumed (without checking) that the other log is the winscp.exe log I've asked for (whatever you actually mean by "winscp.exe" – the GUI?). So please post that.

I posted 2 files last time. WinSCP.log and Session.log. Here is Session.log again. Thanks, Boris

You have posted a debug log file (Session.DebugLogPath. I've asked for a session log file (Session.SessionLogPath).

Hello Martin, Yes, I'm running WinSCP.exe on the same machine. Both log files attached.

Please post session log file (Session.SessionLogPath) as well as WinSCP.exe log. Are you running WinSCP.exe on the same machine?

One of my SFTP host (I have about 20 others and they are working all fine) stopped working after 2 years of success. I'm getting error

Exception: System.TimeoutException: Timeout waiting for WinSCP to respond

and cannot figure out why. I'm calling a WinSCPnet (version 5.21) library from Windows service written on C#. The host is working fine from the WinSCP.exe.