You can post the log as a private attachment.
Though note that I'm quite sure the problem is as I've described above. The logs won't likely change that.

Hi Martin,

Thanks for your response. Due to client security limitations we can't share the complete logs over public forum.
Could you please let us know if we can connect separately if possible.
If yes, please share your contact details we will reach out to you.

Without more details, I can only guess that the address is redirected to another server with different host key on weekends.
You can specify hostkeys of both servers (if they are both legitimate) in your script.
https://winscp.net/eng/docs/scriptcommand_open#hostkey

If you want us to look deeper, please post complete logs from weekdays and weekends.

Hi WinSCP Team,

We have a secured environment were data transfer takes place from Windows 2019 Server to vendor SFTP location and vice-versa.
This connectivity takes place through WinSCP application via automated SAS program which is scheduled every day in interval of 30 minutes on Windows Server. This connectivity was working fine every day at same interval from 2023.
But from past two months only during weekends, intermediately we are getting Host key Authentication failure for which log is attached.(some details are hidden due to client security reasons)

Details for server:
AWS hosted server with internet disabled.
OS name :- Microsoft Windows Server 2019 standard
WinSCP version:- 5.19.4 (In development environment we have upgraded the WinSCP version to 6.1.2 but still we are facing the Authentication failure )

Could you please help us with the solution of this issue.