Ok, I usually edit user crontab, so I become root and issue
Bash:
crontab -e
I've the feeling that /etc/crontab could be overwritten if you update the crontab package, not an expert of crontab but maybe you'd better have a look at possible consequences
Probably he does not have Proxmox reboot every day, but instead someone using the VM and shutting sometime it (the VM ony) down by accident at the end of the working day, then complaining that "does not work" the day after.