This is due to an error in the CUPS configuration, as far as I can tell... I had this same problem, and had to either use "su kprinter" or use lynx running as root, in order to do printer administration.
Today, I found the cause of it - a mistake in the default settings in /etc/cups/cupsd.conf
Look for the line
and change it to
After that, you should be able to log in to http://localhost:631/admin
with your root username and password, as expected.