"Broken FTP clients" is not enabled in pure-ftpd on install/config
Some FTP clients like 'Windows Explorer' automatically logon anonymously. Since anonymous logon in pure-ftpd is disabled by default, pure-ftpd drops the connection and leaves you to thinking that something is wrong with the FTP daemon.
The solution to this is to have the installer enable the -b flag. This can be accomplished by running the following commands:
echo yes > /etc/pure-ftpd/conf/BrokenClientsCompatibility /etc/init.d/pure-ftpd-mysql restart