To use the administration with the Web front-end (CUPS) or the printer administration tool (KDE), the user root must be set up as CUPS administrator with the CUPS administration group sys and a CUPS password. Do this as root with the following command:
lppasswd -g sys -a root
If this is not done, administration with the Web interface or with the administration tool is not possible, because the authentication fails if no CUPS administrator has been configured. Instead of root, any other user can also be appointed as CUPS administrator. (Note : use strong-type password, minimal length : 6).