Answered: Error SSH: Rebex.Net.SshException: The connection was closed by the server.

Hello, 12.3.52.138:990 is not an SFTP/SSH server. It's an FTP server implicit secured with TLS. Please use Rebex FTP/SSL to connect to this server:

var ftp = new Ftp();
ftp.Connect("12.3.52.138", 990, SslMode.Implicit);

Alternatively, you might use FileTransferClient (part of Rebex File Transfer Pack) which (like FileZilla) supports both FTP and SFTP:

var client = new FileTransferClient();
client.Connect("12.3.52.138", 990, FileTransferMode.FtpSslImplicit);

If you get an exception indicating that the server certificate was rejected, check out this blog post: https://blog.rebex.net/howto-server-certificate-rejected-exception/