That version of Rebex SFTP has been released 10 years ago and is no longer suitable for use with current SFTP servers. Many servers now require the client to use SSH ciphers that have not even existed in 2007.
Upgrading to the current version should fix the issue. To make sure it works, give the current trial version a try.
If you are interested in details, see the list of currently supported ciphers.
For comparison, this is the list of ciphers we supported in 2007:
Key exchange algorithms:
Host key algorithms: