My client is your client, and they recently stopped being able to upload files to us. Also recently we upgraded our servers to use OpenSSH 6.8 and it seems to have been the issue with other client's software. So I wanted to start troubleshooting by asking simply, does/can this software support OpenSSH 6.8? Below are their error messages.
sshconnect.c:373: 09/17/15-12:07:00 Connecting to host(s) <sftp.our.site.com>
sshconnect.c:390: 09/17/15-12:07:00 Number of hosts = 1
sshconnect.c:393: 09/17/15-12:07:00 Connecting to SSHFTP server at:sftp.our.site.com port:22
log.c:460: 09/17/15-12:07:01 no kex alg