Cannot use scanner of HP LaserJet Pro MFP M28w on Debian 9.6

Hello, my name is Julian and I got a HP LaserJet Pro MFP M28W which refuses to scan. I hope that you can help my out.

I'm running Debian 9.6 with xfce. Printing works fine via USB and via Wifi.

My installation routine:
1. Purged hplip and libhpmud0 coming from Debian repositories from my system (They are too outdated to support my printer model)
2. Installed hplip 3.18.12 via automated script

After this I can setup the printer just fine. What I noticed, is that I get no systray icon.

Output of "hp-check -t" seems to be ok:

---------------
| SYSTEM INFO |
---------------

 Kernel: 4.9.0-8-amd64 #1 SMP Debian 4.9.130-2 (2018-10-27) GNU/Linux
 Host: Denkeinlage
 Proc: 4.9.0-8-amd64 #1 SMP Debian 4.9.130-2 (2018-10-27) GNU/Linux
 Distribution: 2 9.6
 Bitness: 64 bit

-----------------------
| HPLIP CONFIGURATION |
-----------------------

HPLIP-Version: HPLIP 3.18.12
HPLIP-Home: /usr/share/hplip
HPLIP-Installation: Auto installation is supported for debian distro 9.6 version

Current contents of '/etc/hp/hplip.conf' file:
# hplip.conf. Generated from hplip.conf.in by configure.

[hplip]
version=3.18.12

[dirs]
home=/usr/share/hplip
run=/var/run
ppd=/usr/share/ppd/HP
ppdbase=/usr/share/ppd
doc=/usr/share/doc/hplip-3.18.12
html=/usr/share/doc/hplip-3.18.12
icon=/usr/share/applications
cupsbackend=/usr/lib/cups/backend
cupsfilter=/usr/lib/cups/filter
drv=/usr/share/cups/drv/hp
bin=/usr/bin
apparmor=/etc/apparmor.d
# Following values are determined at configure time and cannot be changed.
[configure]
network-build=yes
libusb01-build=no
pp-build=no
gui-build=yes
scanner-build=yes
fax-build=yes
dbus-build=yes
cups11-build=no
doc-build=yes
shadow-build=no
hpijs-install=no
foomatic-drv-install=no
foomatic-ppd-install=no
foomatic-rip-hplip-install=no
hpcups-install=yes
cups-drv-install=yes
cups-ppd-install=no
internal-tag=3.18.12
restricted-build=no
ui-toolkit=qt5
qt3=no
qt4=no
qt5=yes
policy-kit=no
lite-build=no
udev_sysfs_rules=no
hpcups-only-build=no
hpijs-only-build=no
apparmor_build=yes
class-driver=no

Current contents of '/var/lib/hp/hplip.state' file:
Plugins are not installed. Could not access file: No such file or directory

Current contents of '~/.hplip/hplip.conf' file:
[installation]
date_time = 02/01/19 16:42:24
version = 3.18.12

[upgrade]
notify_upgrade = true
last_upgraded_time = 1546439851
pending_upgrade_time = 0

 <Package-name> <Package-Desc> <Required/Optional> <Min-Version> <Installed-Version> <Status> <Comment>

--------------
| COMPILEDEP |
--------------

 gcc gcc - GNU Project C and C++ Compiler REQUIRED - 6.3.0 OK -
 make make - GNU make utility to maintain groups of programs REQUIRED 3.0 4.1 OK -
 libtool libtool - Library building support services REQUIRED - 2.4.6 OK -

------------------------
| General Dependencies |
------------------------

 pyqt5-dbus PyQt 5 DBus - DBus Support for PyQt5 OPTIONAL 5.0 5.7 OK -
 libcrypto libcrypto - OpenSSL cryptographic library REQUIRED - 1.1.0 OK -
 python-xml Python XML libraries REQUIRED - 2.2.0 OK -
 libnetsnmp-devel libnetsnmp-devel - SNMP networking library development files REQUIRED 5.0.9 5.7.3 OK -
 sane-devel SANE - Scanning library development files REQUIRED - - OK -
 pil PIL - Python Imaging Library (required for commandline scanning with hp-scan) OPTIONAL - 1.1.7 OK -
 libpthread libpthread - POSIX threads library REQUIRED - 2.24 OK -
 python-devel Python devel - Python development files REQUIRED 2.2 2.7.13 OK -
 cups-devel CUPS devel- Common Unix Printing System development files REQUIRED - 2.2.1 OK -
 python-dbus Python DBus - Python bindings for DBus REQUIRED 0.80.0 1.2.4 OK -
 cups-ddk CUPS DDK - CUPS driver development kit OPTIONAL - - OK -
 reportlab Reportlab - PDF library for Python OPTIONAL 2.0 3.3.0 OK -
 pyqt5 PyQt 5- Qt interface for Python (for Qt version 4.x) REQUIRED 5.0 5.7 OK -
 libusb libusb - USB library REQUIRED - 1.0 OK -
 cups-image CUPS image - CUPS image development files REQUIRED - 2.2.1 OK -
 python2X Python 2.2 or greater - Python programming language REQUIRED 2.2 2.7.13 OK -
 python-notify Python libnotify - Python bindings for the libnotify Desktop notifications OPTIONAL - - OK -
 libjpeg libjpeg - JPEG library REQUIRED - - OK -
 sane SANE - Scanning library REQUIRED - - OK -

----------------------
| Scan Configuration |
----------------------

 scanext Scan-SANE-Extension REQUIRED - 3.18.12 OK -
 hpaio HPLIP-SANE-Backend REQUIRED - 3.18.12 OK 'hpaio found in /etc/sane.d/dll.conf'

-------------------------
| External Dependencies |
-------------------------

 gs GhostScript - PostScript and PDF language interpreter and previewer REQUIRED 7.05 9.26 OK -
 scanimage scanimage - Shell scanning program OPTIONAL 1.0 1.0.25 OK -
 cups CUPS - Common Unix Printing System REQUIRED 1.1 2.2.1 OK 'CUPS Scheduler is running'
 network network -wget OPTIONAL - 1.18 OK -
 policykit PolicyKit - Administrative policy framework OPTIONAL - 0.105 OK -
 xsane xsane - Graphical scanner frontend for SANE OPTIONAL 0.9 0.999 OK -
 dbus DBus - Message bus system REQUIRED - 1.10.26 OK -
 avahi-utils avahi-utils OPTIONAL - 0.6.32 OK -

---------------------
| Python Extentions |
---------------------

 hpmudext IO-Extension REQUIRED - 3.18.12 OK -
 cupsext CUPS-Extension REQUIRED - 3.18.12 OK -

------------------------------
| DISCOVERED SCANNER DEVICES |
------------------------------

device `hpaio:/usb/HP_LaserJet_MFP_M28-M31?serial=VNC3C78580' is a Hewlett-Packard HP_LaserJet_MFP_M28-M31 all-in-one

--------------------------
| DISCOVERED USB DEVICES |
--------------------------

  Device URI Model
  -------------------------------- -----------------------
  hp:/usb/HP_LaserJet_MFP_M28-M31? HP LaserJet MFP M28-M31
  serial=VNC3C78580

---------------------------------
| INSTALLED CUPS PRINTER QUEUES |
---------------------------------

HP-LaserJet-MFP-M28-M31
-----------------------
Type: Printer
Device URI: hp:/usb/HP_LaserJet_MFP_M28-M31?serial=VNC3C78580
PPD: /etc/cups/ppd/HP-LaserJet-MFP-M28-M31.ppd
PPD Description: HP LaserJet MFP m28-m31, hpcups 3.18.12
Printer status: printer HP-LaserJet-MFP-M28-M31 is idle. enabled since Wed 02 Jan 2019 16:42:19 CET
Communication status: Good

--------------
| PERMISSION |
--------------

USB HP-LaserJet-MFP-M28-M31 Required - - OK Node:'/dev/bus/usb/001/005' Perm:' root lp rw- rw- rw- rw- rw- r--'
No errors or warnings.

Done.

As soon as I try to scan, the installation of a plugin is started (although the list of supported printers suggests that no plugin is needed for this model...). The verification fails, but I install it anyway. It ends with "Plug-in installation successful", directly followed by "error: Failed to install Plug-in. Either you have chosen to skip the Plug-in installation or entered incorrect Password."

scanimage -L detects the scanner:
device `hpaio:/usb/HP_LaserJet_MFP_M28-M31?serial=VNC3C78580' is a Hewlett-Packard HP_LaserJet_MFP_M28-M31 all-in-one

Now whenever I try to scan (hp-scan or xsane), the scanner actually performs a scan, but after finishing all I get is this error message:
error: SANE: Error during device I/O (code=9)

In syslog I get this suspicious line:
hp-scan: http.c 157: read_stream error stat=49
which according to the source stands for HPMUD_R_IO_TIMEOUT. I'm not sure if my skills are sufficient to dig further into the code...

Thanks in advance and greetings from Berlin,
Julian