I saw the following entry in Google just a couple down from this thread when I googled the same error.
CUPS Software Administrators Manual
The AuthGroupName directive sets the group to use for Group authentication.
The AuthGroupName directive must appear inside a Location directive. ...
- 251k - Cached - Similar pages
This is a quote from the AuthGroupName section of the CUPS manual:
"The AuthGroupName directive must appear inside a Location directive."
I moved it to the admin section the error has disappeared.
Now I get:
"Unable to open /etc/cups/classes.conf - No such file or directory"