After several hours Goolgling, I found the problem: not all the required dependencies of HPLIP are installed in Waldorf, and the required user permissions are not set.
To check if HPLIP is set up correctly, run:
sudo hp-check -tIf there are unmet dependencies or permission issues, run the required fix to install and add automatically.
My symptoms were:
Could see printer in System>Printers>Configure Printers but not use.
Could see printer in device setup HPLIP gui (hp-toolbox) but not install.
Could install printer via hp-setup as root, but could not print from inside web pages (could print a text file) and could not administer printer from Device Manager.