This error comes from your FTP server when requesting size of the file.
Can you try using this code before downloading the file:
using(Ftp ftp = new Ftp())
{
ftp.ConnectSSL("ftp.server.com");
ftp.Login("user", "password");
ftp.Configuration.DontGetFileSizeBeforeDownload = true;
ftp.Download("report.txt", @"c:\report.txt");
ftp.Close();
}