I'm posting this here to potentially help others. A customer reported an SSH/SFTP login failure, yet was passing the correct login/password, AND other programs seemed to work (such as logging in via a terminal).
The problem turned out to be a limit on the number of SSH connections allowed by the server. The server was configured to only allow 1 connection per user except for root. Every time he used Chilkat, there was already one connection existing (the terminal) for the user account, and therefore Chilkat failed. But when trying with the terminal, his app wasn't running and therefore there was no existing connection and the terminal login succeeded.