Hi LL,
Apparently this is a clear issue of SSL handshake failure, and the client server failed to reach a mutually agreed protocol to use for the SFTP connection. If it’s possible for you to actually enable more detail debug logs like telling what are the SSL protocol negotiated between the client and the server, it would also help to understand more. Also getting info from the log of SFTP server side will help to unearth more details. I assumed this likely to be a configuration issue than an actual product defect.
I would also suggest to get this book if possible, as it’s quite handy to understand SSL/TLS related matters in details.