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If the number from the message is a very large number then the problem is typically caused by a message printed from some profile/logon script. It violates an 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, these 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 the text. Once you find it, move the command to a 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. Typically you will need to move the commands from .bashrc script to .bash_profile.
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 an SFTP server, or encounter a fatal error. You should contact your server administrator.
Another possibility is that the server is configured to only allow an SCP protocol and not the SFTP protocol, in such a way that SCP fallback mechanism of WinSCP does not work. The solution is to explicitly choose SCP protocol on the Login dialog.