Here's a workaround solution thanks to https://bugs.launchpad.net/ubuntu/+source/proftpd-dfsg/+bug
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.
/etc/init.d/proftpd
Find this line:
start-stop-daemon --stop --signal $SIGNAL --quiet --pidfile "$PIDFILE"
Change to this:
start-stop-daemon --stop --signal $SIGNAL --retry 1 --quiet --pidfile "$PIDFILE"
This change solved it for me.
Suggestions for improvements are most welcome.