upstart : after a random uptime service failed to start - unable to granpt: No such file or directory

[Goacid]

Kernel : latest 3.14.20 + grsec

Description: Ubuntu 14.04.1 LTS
Release: 14.04
Upstart version : Upstart : 1.12.1-0ubuntu4.2
Problem reported on more than 10 servers with the same configuration

I use upstart to start and stop my service. After a random time (for minutes to days), some services begin failed to start, some other continue starting but always put a new line in dmesg

Example for rsyslogd service which never success to start when the problem begin :
service rsyslog restart
stop: Unknown instance:
start: Job failed to start
dmesg
[...]
init: Failed to spawn rsyslog pre-start process: unable to granpt: No such file or directory
init: Failed to spawn startpar-bridge (rsyslog--stopped) main process: unable to granpt: No such file or directory
init: Failed to spawn rsyslog pre-start process: unable to granpt: No such file or directory
init: Failed to spawn startpar-bridge (rsyslog--stopped) main process: unable to granpt: No such file or directory
init: Failed to spawn rsyslog pre-start process: unable to granpt: No such file or directory
init: Failed to spawn startpar-bridge (rsyslog--stopped) main process: unable to granpt: No such file or directory

And for ssh, which success :
service ssh restart
ssh stop/waiting
ssh start/running, process 32052
dmesg
[...]
tinit: Failed to spawn startpar-bridge (ssh--stopped) main process: unable to granpt: No such file or directory
init: Failed to spawn startpar-bridge (ssh--started) main process: unable to granpt: No such file or directory

The only solution I found is to reboot servers, until next time the problem appeared.