simple-scan does not communicate with scanner, also it is found with "lsusb" and "scanimage -L"

[bel]

It seems simple-scan can no longer communicate with my scanner, although it was able to scan with the same hardware before. The hardware is found, but simple-scan reports communication errors. The hardware itself works.

user@host:$ lsusb
Bus 002 Device 002: ID 058f:9380 Alcor Micro Corp. Flash Drive
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 008 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 007 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 005: ID 04a9:176d Canon, Inc. PIXMA MG2550
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub

<turned printer off>

user@host:$ lsusb
Bus 002 Device 002: ID 058f:9380 Alcor Micro Corp. Flash Drive
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 008 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 007 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub

<turned printer on again>

user@host:$ sudo scanimage -L
[sudo] Passwort für user:
device `pixma:04A9176D_1B4C43' is a CANON Canon PIXMA MG2500 Series multi-function peripheral

<tried some hints from some forum>

user@host:$ groups
user adm cdrom sudo dip plugdev lpadmin sambashare

user@host:$ sudo adduser user lpadmin
Der Benutzer »user« ist bereits ein Mitglied der Gruppe »lpadmin«.

user@host:$ sudo chmod o+wr /dev/bus/usb/00*/*

<restated simple-scan>

==> still the same error

user@host:$ ubuntu-bug -w
Gtk-Message: GtkDialog mapped without a transient parent. This is discouraged.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: simple-scan 3.20.0-0ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-64.85-generic 4.4.44
Uname: Linux 4.4.0-64-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Sep 2 08:48:09 2017
ExecutablePath: /usr/bin/simple-scan
InstallationDate: Installed on 2014-11-24 (1012 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
MachineType: Hewlett-Packard HP Compaq dc7900 Convertible Minitower
ProcEnviron:
 PATH=(custom, username)
 SHELL=/bin/bash
 LANG=de_DE.UTF-8
 LANGUAGE=de_DE
 XDG_RUNTIME_DIR=<set>
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.4.0-64-generic root=/dev/mapper/ubuntu--vg-root ro quiet splash vt.handoff=7
SourcePackage: simple-scan
UpgradeStatus: Upgraded to xenial on 2016-11-07 (298 days ago)
dmi.bios.date: 03/05/2009
dmi.bios.vendor: Hewlett-Packard
dmi.bios.version: 786G1 v01.16
dmi.board.asset.tag: CZC9368N0K
dmi.board.name: 3032h
dmi.board.vendor: Hewlett-Packard
dmi.chassis.asset.tag: CZC9368N0K
dmi.chassis.type: 6
dmi.chassis.vendor: Hewlett-Packard
dmi.modalias: dmi:bvnHewlett-Packard:bvr786G1v01.16:bd03/05/2009:svnHewlett-Packard:pnHPCompaqdc7900ConvertibleMinitower:pvr:rvnHewlett-Packard:rn3032h:rvr:cvnHewlett-Packard:ct6:cvr:
dmi.product.name: HP Compaq dc7900 Convertible Minitower
dmi.sys.vendor: Hewlett-Packard

[bel (bel2125)]

Screenshot of the error shown by the application