SFTP error: "Failed to send key exchange initialization message"

[Mr X]

When I attempt to create an SFTP connection to my local SFTP server (which uses the Wing FTP Server software, which is a competent SFTP server software despite the name) I only get the following error message:

"Failed to send key exchange initialization message"

After trying a few times, I often get the following error message instead:

"recvfrom failed: ECONNRESET (Connection reset by peer)"

At one time I also got the error message:

"The connection did not complete"

All this happens when I use the "Test connection" button from the account setup screen (and the account also fails when I try to use it for a sync, not getting a directory listing to choose folders from).

All other well-known SFTP clients connect just fine to the server, both before, during and after the tests with FolderSync from the phone (Nexus 4).

What can cause this? Probably some incompatibility with Wing SFTP Server, right?

I can make available an account on the server over the internet if you (i.e. the developer of FolderSync) would like to do some debugging of this issue?