I have a ruby app built in to a Java jar file. When the app connects via sftp the connection is immediately terminated by the server (CoreFTP server 589.38, but same happens with older versions, running on Windows Server 2008 R2).
Using any other sftp server on the Windows box works (e.g. cerberus).
Connecting via command line (ubuntu) to coreftp from the machine running the jar file works.
Client SSH: SSH-2.0-Ruby/Net::SSH_4.1.0
I can send along a wireshark packet capture if necessary.