/var/log/messages で proftpd からのエラーを確認してください。
単純な FTP セッションの /var/log/messages は次のとおりです。
Jan 16 16:39:15 localhost xinetd[577]: START: ftp pid=9317 from=::ffff:10.50.2.4
Jan 16 16:39:16 localhost proftpd[9317]: 10.52.52.203 (10.50.2.4[10.50.2.4]) - FTP session opened.
Jan 16 16:39:16 localhost proftpd[9317]: 10.52.52.203 (10.50.2.4[10.50.2.4]) - FTP session closed.
Jan 16 16:39:16 localhost xinetd[577]: EXIT: ftp status=0 pid=9317 duration=1(sec)
Jan 16 16:39:16 localhost xinetd[577]: START: ftp pid=9318 from=::ffff:10.50.2.4
Jan 16 16:39:16 localhost proftpd[9318]: 10.52.52.203 (10.50.2.4[10.50.2.4]) - FTP session opened.
Jan 16 16:39:22 localhost proftpd[9318]: 10.52.52.203 (10.50.2.4[10.50.2.4]) - Preparing to chroot to directory '/var/www/vhosts/domain.tld'
Jan 16 16:40:03 localhost xinetd[577]: Exiting...