This error message is specific to the ProFTPd server. A Chilkat customer found this error within the LastErrorText after trying to upload or download a file, or retrieve a directory listing:
425 Unable to build data connection: Operation not permitted
The latest ProFTPd server has a configuration setting that by default requires SSL/TLS sessions to be re-used, and this breaks *many* FTP clients, including Chilkat. The fix to be made on the server is to add the following line to the proftpd.conf and then restart it.