The ProFTPD is killed quit frequently every night.

Dec 03 23:59:02 www.walkmen.net proftpd[xxxxx] localhost: ProFTPD killed (signal 15)
Dec 03 23:59:02 www.walkmen.net proftpd[xxxxx] localhost: ProFTPD x.y.z standalone mode SHUTDOWN

This is happened to be a bug when stopping ProFTPD too fast. The way to fix it is to modify the /etc/init.d/proftpd script and change every line having

start-stop-daemon --stop --signal $SIGNAL --quiet --pidfile "$PIDFILE"

to

start-stop-daemon --stop --signal $SIGNAL --retry 1 --quiet --pidfile "$PIDFILE"


Source: http://stackoverflow.com/questions/23666697/proftpd-killed-signal-15-error-how-to-fix-logrotate-restart-error