There are some cases you having issue to connect FTP using PureFTPD. The easiest way is to change it to ProFTPD.
[cc]cd /usr/local/directadmin/custombuild
./build update
./build set ftpd proftpd
./build proftpd[/cc]
Make sure set correct path for the access and auth log in its configuration.
[cc]vim /etc/proftpd.conf[/cc]
ExtendedLog /var/log/proftpd/access.log WRITE,READ write
ExtendedLog /var/log/proftpd/auth.log AUTH auth