Same problem here with www-data (uid 33) when try FTP login:
Answer: 331 User demo2 OK. Password required
Command: PASS **************
Answer: 530 Sorry, but I can't trust you
I'v found this: http://marc.info/?l=pureftpd-list&m=127448475305253
So I'v crated a new site, then a new ftp user, and modifed the User identifier (UID) to "ftp-ro" + Group identifier (GID): "ftp-ro"
Then I modified the group of the directory to ftp-ro (chown -R .ftp-ro directory_name), and modified group priviliges to setgid (chmod g+s directory_name) at command line.
With setgid the group will be ftp-ro for all of the new files and subdirectories.
I hope it will help to others who find this thread.