Scanner is only detected with root user, printer cannot be added through GNOME

[divoplade <d@divoplade.fr>]

Dear guix,

When running simple-scan, 

$ LANG=C simple-scan

I get a dialog showing "No scanners detected".

However, when I run the same with sudo,

$ LANG=C sudo simple-scan

I don’t get the icons, but the scanner is detected and usable.

I am currently in the following supplementary-groups:

'("wheel" "netdev" "audio" "video" "lpadmin" "scanner")

Is it not enough?

Running: 
$ hp-scan -ldebug

I get:
HP Linux Imaging and Printing System (ver. 3.20.11)
Scan Utility ver. 2.2

Copyright (c) 2001-18 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

hp-scan[9905]: debug: getDeviceUri(None, None, ['hpaio'], {'scan-type': 
(<built-in function gt>, 0)}, , True)
hp-scan[9905]: debug: Mode=0
error: No device selected/specified or that supports this
functionality.


Also, when I try to add it as a printer in the GNOME control center, I
can select the printer but then I get a dialog "Failed to add new
printer.", with a console message that seems relevant: 

(gnome-control-center:9651): printers-cc-panel-WARNING **:
21:08:07.779: cups-pk-helper: addition of printer Deskjet-2050-J510
failed: cups-authentication-canceled

What do I do wrong? What can I do to debug it?

Best regards,

divoplade