1. What is the default username and password for CUPS web interface? I tried my root username and password, and my own username and password; neither of them worked.

2. How do I add a printer so that it can work with the HPLIP Toolbox? (Yes, it is an HP printer.)

In other words, you should add a user to the cups user database using
lppasswd -a <username>

I did as suggested in the manual, but it still rejects the username and password I selected (yes, I did run it as sudo, and it did not report any problem with the password, which was over six characters, containing at least one letter and one number).

Not to be an annoying thread-bumper... but this problem is really stumping me... the Gnome control panel for printers works fine, but the CUPS Web Interface is broken for me (see above)... :\

I have been having the same issue, and I can't get an HP 8450 to work. After doing some searching on the internet, I have found that for some security reason Ubuntu package maintainers have disabled the web interface. So they say that you are only to use the gnome printer install interface. However, there seem to be issues with the gnome one not doing what I want it to do either. I hope you find a way to get your printer installed under Ubuntu with the Gnome interface. Let me know if you get a solution using the Gnome interface for your issue.

mowestusa