After all the attempts, I now have one error every 5 minutes, each time munin triggers, and receive this mail to the munin reporting address:
subject: Cron <munin@b3> if [ -x /usr/bin/munin-cron ]; then /usr/bin/munin-cron; fi body: not a reference at /usr/share/perl5/Munin/Master/Utils.pm line 901 Luc -- To UNSUBSCRIBE, email to [email protected] with a subject of "unsubscribe". Trouble? Contact [email protected]

