This error is raised when a 30-day trial key has expired. It is supposed to inform the trial user that their evaluation period is over. If this is not the case, something is wrong somewhere - common reasons include wrong system date, or custom application setting multiple different trial keys (some of them expired).
Could you please try running the following simple console application using Rebex FTP? Does it work, or does it fail with "trial version expired" error as well?
static void Main()
Licensing.Key = "==AN4iE2sf569ak96g8Ov9+8z2bTRnRoS3JieHn1m7Zwas==";
using (var ftp = new Ftp())
ftp.LogWriter = new ConsoleLogWriter(LogLevel.Debug);