If you are running ProFTPD on Ubuntu 14.04, you may have noticed the server is being stopped weekly. Upon further investigation it turns out that the error seems to be happening when log rotation is taking place. If this is indeed the problem you are having then the log will reflect the following.

... ProFTPD killed (signal 15)

… ProFTPD 1.3.5rc3 standalone mode SHUTDOWN

The problem is due to ProFTPD not stopping in time to be restarted. The workaround is to edit the service file to add a retry and sleep to /etc/init.d/proftpd, see below.

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

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