ProFTPD on Ubuntu is being killed unexpectedly.
$ tail /var/log/proftpd/proftpd.log.1
... ProFTPD killed (signal 15)
... ProFTPD 1.3.5rc3 standalone mode SHUTDOWN
Diagnosis: the time of the message is the same time that
cron runs the
Google searches turn up various info about ProFTPD having a glitch on Debian-based systems due to log files, log rotations, and the ProFTPD server not restarting.
I will add a bounty for a solution to this. I’ve posted a workaround answer too, in case it’s helpful for others.