RESOLUTION
Make sure that 21 port is opened on the server:nmap -p21 -P0 SERVER_IP Check /var/log/messages file for any FTP related errors. If you are getting a PAM authentication error you need to disable PAM in /usr/local/psa/ftpd/etc/proftpd.conf file. For RPM Plesk version this file is located in /etc/ directory.
AuthPAM off
Make sure that inetd/xinetd is running. If inetd/xinetd is not running you also may be experiencing problems with sending mail (SMTP).
ps -aux | grep inetd or
ps -aux | grep xinetd If inetd/xinetd is not running it must be started.
for Linux servers:
/etc/rc.d/init.d/xinetd start for FreeBSD servers:
/usr/sbin/inetd -wW