permission denied for editing sudoers file
Try changing the permission for the sudoers file by following command:
sudo chown -R username:username sudoers
chmod 755 sudoers
It is not a problem of the sudoers configuration file. You can check the
configuration of your file using visudo -c. If you run against your file, you
will check that it is parsed OK. I can't identify the problem with only the
info you provided, but here are some things you can try.
1. Make sure that all the path /etc/sudoers is executable
2. Make sure that the / directory is with permissions 755(drwxr-xr-x)
3. Try to reconfigure the package with the default values running
dpkg-reconfigure as root
4. Please provide info about the new permissions on /.
PS: I find weird is that your sudoers file has write permissions, remember you
should only edit the sudoers file with visudo.