Intermittent SFTP issues with Hyper-V

Hi WinSCP Support Team,
We have been using your WinSCP software reliably for many years and it is a great product. We are using 5.17.2 and 5.21.6.
However we have come across an issue that is difficult to pin down and it is the same on all versions of WinSCP that we have tested. Here is the explanation.
We do SFTP transfers over port 22 to a customer. These are through Windows Server 2008 R2 and Windows Server 2019 boxes. Everything was great until we migrated to a Virtual Machine environment, which is:

• A Big Windows Donk sliced into 4 Virtual Machines using Microsoft Hyper-V on Windows Server 2019 Standard.
  
• These are named VM01, VM-02, VM03 and VM04.
  
• VM01 is our Primary Domain Controller so it runs DNS and DHCP and its DNS is set to 127.0.0.1 while the other VM’s have their DNS set to point to VM01 (normal).
  
• We have no troubles when connecting from VM01 however the other three give intermittent connection errors like this:
  

  2023-01-31 19:00:30.692 Server offered these authentication methods: gssapi-with-mic,password,publickey,keyboard-interactive
  . 2023-01-31 19:00:30.692 Using SSPI from SECUR32.DLL
  . 2023-01-31 19:00:30.692 Trying gssapi-with-mic...
  . 2023-01-31 19:00:30.692 Attempting GSSAPI authentication
  . 2023-01-31 19:00:30.708 GSSAPI authentication initialisation failed
  . 2023-01-31 19:00:30.708 The target was not recognized.
  . 2023-01-31 19:00:30.708 Attempting keyboard-interactive authentication
  . 2023-01-31 19:00:30.723 Prompt (keyboard interactive, "SSH server authentication", "Password Authentication:", "xxxe's password: ")
  . 2023-01-31 19:00:30.723 Using stored password.
  . 2023-01-31 19:00:30.817 Access granted
  . 2023-01-31 19:00:30.817 Opening main session channel
  . 2023-01-31 19:00:30.833 Opened main channel
  . 2023-01-31 19:00:47.234 Waiting for data timed out, asking user what to do.
  . 2023-01-31 19:00:47.234 Asking user:
  . 2023-01-31 19:00:47.234 **Host is not communicating for 15 seconds.
  . 2023-01-31 19:00:47.234 
  . 2023-01-31 19:00:47.234 Wait for another 15 seconds?** ()
  . 2023-01-31 19:00:58.970 Attempt to close connection due to fatal exception:
  * 2023-01-31 19:00:58.970 **Terminated by user.**
  . 2023-01-31 19:00:58.970 Closing connection.

This happens about 15% of the time on servers VM02 to VM04. I wonder if VM01 is working because it accesses the LAN card directly or in some way that is different to VM02..VM04?

We tried all sorts of WinSCP config variations starting with Optimize Connection buffer, no change. We are generally using scripting/batch files but get the same problem when using the GUI. We can use the .NET assembly but I am not confident it will be any different to the scripting we do now.

Do you have any suggestions on what we might do to solve this intermittent connection issue? It has caused us to revert back to old servers which is a nuisance.

Many thanks for your assistance.

Cheers
James Treacy
Senior Applications Development Manager
The Dataline Group
Sydney Australia
Email james.treacy@dataline.com.au

Thanks for your report.
Does the timeout occur always this way right after the authentication?
Can you enable Debug 2 level logging and post new logs? Maybe even like two or three logs of different failed sessions and some logs of successful sessions.