FTP is not working and has the following error:
421 Service not available, remote server has closed connection

Also, in the message logs:
Plesk Unable to FTP – missing scoreboard file (Linux)

On CentOS 6 and 7 if you experience issues logging on to FTP server please check the logs for the following:

# tail -f /var/log/messages

Look for this error:

proftpd[11951]: fatal: ScoreboardFile: : unable to use '/var/run/proftpd/scoreboard': No such file or directory on line 74 of '/etc/proftpd.conf'

Check in /var/run for a proftpd folder, if it’s missing you have a bug in your ProFTPd version usually caused by using the Atomic repo to update ProFTPd.

Check for atomic:

# ls -la /etc/yum.repos.d/ | grep atomic
-rw-r--r--.   1 root root   827 May 18  2016 atomic.repo

The solution is to remove the package and reinstall from Plesk repo:

rpm -e --nodeps psa-proftpd
plesk installer --select-release-current --install-component=proftpd
service xinetd restart

Then you should be able to connect again. Please note that any changes to proftpd.conf such as enabling custom PassivePorts will be lost and need to be recreated.

Source: https://community.webcore.cloud/tutorials/plesk_articles/plesk_unable_to_ftp_missing_scoreboard_file_linux/