If you see the webinterface, but cant login to some options, please run these comands again:
Add the cupsys user to the shadow group:
adduser cupsys shadow
and restart the cups daemon:
If you cant reach the webinterface at all, please check if you set the options in /etc/cups/cupsd.conf in the correct sections. The option "AuthGroupName shadow" must be right below or above the AuthGroupName option that is commented out in the file.