Are you used to redirecting stderr and stdout in a cron job and having it work seven ways to Sunday, but are having issues with Ubuntu doing the same?
It might just be an issue with the chosen shell "dash" instead of the old bash shell.
Try appending this instead of the old bash shell tricks:
command-to-run > /dev/null 2>&1
This is will be my dumping spot for other Ubuntu strangeness.
Neuter services (runlevel stuff):
update-rc.d -f clamav-daemon remove update-rc.d -f clamav-freshclam remove
Same under systemd strangeness:
systemctl disable clamav-daemon.socket systemctl disable clamav-daemon.service systemctl disable clamav-freshclam.service
Check for corrupt packages: - or just see what others are up to :-)