Thank you for the log file. It showed that the error is misleading and is caused by a server side bug very probably.
The error is response to
Command: MLST /filename.zip
Response: 521 Data connection cannot be opened with this PROT setting.
MLST command does not open Data connection. It provides information about requested file. For details see RFC 3659.
We also use Pure-FTPd server for our tests. Our server is able to handle
MLST command successfully. It seems that you have an old version of the server.
The ideal solution is to upgrade your server to the newer version. The
MLST command is standard functionality nowadays.
If you cannot upgrade the FTP server, you can try to workaround the issue by disabling
MLST on the client. It can be done like this:
var client = new Rebex.Net.Ftp();
client.EnabledExtensions &= ~FtpExtensions.MachineProcessingList;