PHP CLI "Out of memory" error on VPS server running CentOS with cPanel
It turns out that following an unwanted cPanel update my SSH port was changed from the default value and I was unable to login. I restarted SSH in safe mode in order to login.
Today, on a hunch, I looked at the SSH port in /etc/ssh/sshd-config, restarted SSH in "normal" mode, and logged in with the correct port: the scripts now run perfectly.
So the cause was using SSH in safe mode, just that. Hopefully this very specific problem will be useful to someone else: never use SSH safe mode for daily use unless you absolutely need to.