Try logging as root:
sudo su
Then do the following:
chmod 644 /usr/lib/sudo/sudoers.so
chown -R root /usr/lib/sudo
This should re-establish the correct privileges.
Try logging as root:
sudo su
Then do the following:
chmod 644 /usr/lib/sudo/sudoers.so
chown -R root /usr/lib/sudo
This should re-establish the correct privileges.