In order to discover vendor and version of the software use telnet to connect to the ssh port on the server and it will tell you. For instance, this is what I get from my workstation:
in reply to Re^2: Net SFTP Foreign rget
in thread Net SFTP Foreign rget
$ telnet localhost 22
Connected to localhost.localdomain.
Escape character is '^]'.
Also, can you dump the result of calling stat on any of those files that fail to transfer: