When an FTP client such as Rebex FTP/SSL intends to establish a secure connection using explicit TLS/SSL, it connects to the server without using any encryption and issues AUTH TLS or AUTH SSL command. When both of these commands fail, it means it's not possible to establish an explicit TLS/SSL connection, and an exception is thrown based on the error message received from the server - this is the error you see.
This means that the client might be trying to connect to an FTP server that does not support TLS/SSL (or that it only supports implicit TLS/SSL.