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Re: SFTP connection error - Server unexpectedly closed network connection

What does the server-side log say? Also have your network team check your firewalls.

SFTP connection error - Server unexpectedly closed network connection

Hi there,

I encountered an intermittent error when trying to connect to a client's SFTP server using WinSCP .NET dll. It works for some of the time but failed for most of the cases. Our client said no changes made on their FTP server side. I also updated WinSCP dll to the latest version 5.19.1 but still got the same error. Could you please kindly advise? Any reply is greatly appreciated!
. 2021-07-08 16:41:08.198 --------------------------------------------------------------------------
< 2021-07-08 16:41:08.198 Script: Searching for host...
. 2021-07-08 16:41:08.199 Looking up host "" for SSH connection
. 2021-07-08 16:41:08.213 Connecting to port 22
< 2021-07-08 16:41:08.277 Script: Connecting to host...
. 2021-07-08 16:41:08.277 We claim version: SSH-2.0-WinSCP_release_5.19.1
. 2021-07-08 16:41:08.340 Remote side unexpectedly closed network connection
. 2021-07-08 16:41:08.340 Knocking FTP port.
. 2021-07-08 16:41:08.408 FTP port opened, will suggest using FTP protocol.
< 2021-07-08 16:41:08.409 Script: Server unexpectedly closed network connection.
< 2021-07-08 16:41:08.409 The server rejected SFTP connection, but it listens for FTP connections.
< 2021-07-08 16:41:08.409 Did you want to use FTP protocol instead of SFTP? Prefer using encryption.