The source seems to suggest it can't send the request at all. Have you tried sftp / FileZila to this machine from the machine that you have the ESB server on? I did a little googling on this error, and it seemed like most of the cause was that someone had changed the sftp configuration but had not restarted the sftp server.
unfortunately I also tried sftp / FileZilla on the ESB machine and it worked fine. This is very disturbing.
The only attribute I don't see you setting in the ftp-message-filter is the passive flag. Is your sftp server operating on a standard sftp port?
The error and the fact that no one else has seen this error suggest that this is a configuration problem.
Maybe I am the first one ;-)
The SFTP Server is operating on standard sftp port 22.
When you use Filezila or sftp are you using a certificate? I don't know the setup of your sftp server, so it's hard to make suggestions.
certificate-name, certificate-url, certificate-passphrase