How could I have dropped out of sudoers? [duplicate]
Solution 1:
After
usermod -G advanced dims
the user dims is only a member of the group advanced but not of any other group. So as he isn't a member of the group admin or sudo he is no longer allowed to use sudo.
To just add a user to another group you need to use the -a switch to usermod, like
usermod -a -G advanced dims