This morning I received an email

Cron Daemon ✆ root@(my full domain name)
6:31 AM (4 hours ago)

to root
/etc/cron.daily/apt:
FATAL -> Failed to fork.

I have absolutely zero idea of what caused this ? how to track such issues ?

It means the cron that checks for system updates failed, possibly due to running out of memory. If you run the logview command in lish you'll be able to see any out of memory errors https://www.linode.com/wiki/index.php/L ... umentation

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