After upgrading from Build 317 to 319 we found our client was no longer able to connect to the CoreFTP SFTP server.
To rule out our client, I also tried winscp and this fails to connect to. However CoreFTP client does work.
If we tick the "enable compression" box under the SFTP options it all works fine, however without this ticked it appears the CoreFTP SFTP server sends a malformed SSH packet during the Key Exchange.