thank you for the verbose log. Actually, the problem is not at the Rebex SFTP client side. From the client point of view your server behaves unreliably. Sometimes it accepts the connection and sometimes it behaves as if it wasn't a SFTP server at all.
You can try it yourself:
if you connect to the normally working SFTP server with Telnet client, you will see a server reply in this format "SSH-xyz". However, in case of your above error, the server does not announce itself as expected and instead it closes the connection. So the only way we can handle it is throw an exception.
From your description and the fact that the error is only raised occasionally and most of the time the SFTP server works correctly, it looks like a temporary server problem that occurs from time to time.
For sure, it would be best to ask the server administrators for a help and report this problem to them. They might not even be aware of such behaviour.
If you cannot solve it at the server side, then a typical way to solve this occasional server issue is to retry the Connect method several times, before finally giving up. It would be good to wait a certain amount of time in your program (something like 30 or 60 seconds). In the meantime it is possible that the server will recover.