Comment 36 for bug 1818629

I experienced same problem.

Running hplip-3.20.5.run installed printer but not scanner.

I had made

    sudo apt install libcanberra-gtk-module libcanberra-gtk3-module

because I had got message "Failed to load module "canberra-gtk-module""

But hp-plugin still did not work.

Steps to repair scanner

Changing /usr/share/hplip/base/validation.py line 45 from "pgp.mit.edu" to "keyserver.ubuntu.com".

Install apparmor utils
    sudo apt install apparmor-utils
Run hplip-plugin
    sudo aa-disable /usr/share/hplip/plugin.py
Run as normal user, not as root:
    hp-plugin

AND NOW XSANE CAN SCAN!

Thanks for all above users for advices.