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.
AstroFloyd's blog 
Same problem AND solution here with HP Photosmart C309g and Mint 17 KDE. Thank you for your suggestion!