If … (from the subject) is a very large number then the problem is typically caused by a message printed from some profile/logon script. It violates the SFTP protocol. Some of these scripts are executed even for non-interactive (no TTY) sessions, so they cannot print anything (nor ask user to type something).

The number … represents the first four bytes read from the server. If your login scripts are printing words, this will be the first four characters cast into a number, and not an SFTP message at all.

To fix the problem find out what command in your login script prints text. Once you find it move the command to the proper interactive script, or remove it entirely. The scripts are usually hidden (their name starts with dot) and are located in your home directory on the server.

There are other possible sources of the message in addition to the profile script - some SSH servers print messages if they are unable to start the SFTP server, or encounter a fatal error. You should contact your server administrator.

Another possibility is that the server is configured to only allow the SCP protocol and not the SFTP protocol, in such a way that SCP fallback mechanism of WinSCP does not work. The solution is to choose SCP protocol on the login dialog.