I installed Kubuntu on my mom’s pc. Everything seemed to work, except scanning using the HP Photosmart C6380 All-in-one printer over the wireless network. Printing with the same device using wifi did work. It turned out I should have installed the printer using hplip, not using the KDE System Settings.
I had installed the HP Photosmart C6380 All-in-one printer using the KDE System Settings / Hardware / Printers setup. Printing worked, but scanning did not. When opening XSane or hp-scan (from hplip), the software reported errors like
error: No device selected/specified or that supports this functionality
The solution was to install the printer using the hp-setup command from the hplip package (after removing the existing printer), rather than using the KDE System Settings. I ssh-ed into the machine and used hp-setup -i for text-based output. Installing the printer this way also configures the scanner, which can then be used by e.g. XSane or hp-scan:
Using device: hpaio:/net/Photosmart_C6300_series?ip=192.168.1.61
This was on Kubuntu 13.10, but I suppose the conclusion holds for other distributions using KDE as well, and perhaps even for other desktop environments.