Usually, error 426 occurs when a data connection is broken or closed improperly. The reasons for this can vary - for example, some versions of ActiveSync were observed to cause this problem quite often and some users reported that to be a common problem (we believe this discussion is also related), although the majority of our customers don't seem to suffer from it. What version of Rebex FTP do you use? A full 426 error message might also provide additional information, and a communication log produced using Ftp object's LogWriter property might help as well.
In case this error occurs when uploading a file even though the file has been fully uploaded, a EnableImproperDataCloseWorkaround workaround (available in the latest release of Rebex FTP) can be enabled to get rid of it:
ftp.Options |= FtpOptions.EnableImproperDataCloseWorkaround;
ftp.Options = ftp.Options Or FtpOptions.EnableImproperDataCloseWorkaround
Regarding active and passive mode: Active mode requires the FTP server to establish a connection to the FTP client. This was perfectly acceptable in the 1980s when there was no need for any firewalls and all FTP servers were connected directly to the Internet. But now this approach is outdated and requires complicated FTP-aware firewalls to be able to function. Many firewall and NAT devices don't even support it at all. ActiveSync doesn't support it either. Many cell phone providers don't support it.
Generally, it only makes sense to use active mode when the network policy at the client is very permissive (=security is weak) and the network policy at the FTP server makes passive mode FTP impossible (which is very rare).