Proftpd shutting down

Since the update of the rootserver to Ubuntu Saucy I've been experiencing strange shutdowns of proftpd. Logfiles only showed :

2013-12-29 06:25:27,355 zulu203 proftpd[18394] zulu203.server4you.de: ProFTPD killed (signal 15)
2013-12-29 06:25:27,457 zulu203 proftpd[18394] zulu203.server4you.de: ProFTPD 1.3.5rc3 standalone mode SHUTDOWN

Server could then be restarted (service proftpd restart) without any warning or error message.

I finally found the root cause : logrotate, when it rotates the logs, restarts the proftpd daemon. To do this, it issues the command

invoke-rc.d proftpd restart

Issuing this command manually on the command line, I see the messages :

* Stopping ftp server proftpd                    [ OK ] 
* Starting ftp server proftpd                    [ OK ] 

but still no daemon running !

I then found followind bug report, which suggests that the start of the daemon comes too soon after the shutdown, and therefore silently fails. The proposed solution is to replace

signal stop 1
start

with

signal stop 1
sleep 2
start

in order to let the daemon shut down properly.