After doing Ubuntu double upgrade (from 10.04 to 10.10, then continue with 10.10 to 11.04), I've got this error while logging in into the system:

exec: 3: /usr/lib/update-notifier/update-motd-cpu-checker: not found
run-parts: /etc/update-motd.d/20-cpu-checker exited with return code 2
run-parts: failed to stat component /etc/update-motd.d/50-landscape-sysinfo: No such file or directory
exec: 3: /usr/lib/update-notifier/update-motd-updates-available: not found
run-parts: /etc/update-motd.d/90-updates-available exited with return code 2
exec: 3: /usr/lib/update-notifier/update-motd-reboot-required: not found
run-parts: /etc/update-motd.d/98-reboot-required exited with return code 2

Apparently, some package were removed during the upgrade process. All I need to do was installed one packaged:

$ sudo apt-get install update-notifier-common

And everything's back to normal afterwards :)