Scanner in Officejet 6700 can't be found by Apps

I'm using 64 bit Ubuntu 12.04 LTS, with HP-LIP Software Version 3.13.11, and Device Manager Version 15.0 (Qt4).
I'm also using the HP System Tray Service. The latter has always acted a little odd, but everything seemed to do its job.

About a month ago I installed a new HP OfficeJet 6700 Premium All-In-One; because the native version of HP-LIP that was installed with Ubuntu 12.04 didn't support some of the features (scanning and such), I upgraded to the 3.13.11 mentioned above.

For about a month, things seemed to work normally. I was able to print with any application I used, and could scan with XSane and internally from GIMP and LibreOffice.

Now, however, the 6700 sometimes doesn't show up at all when I run the HP Device Manager from the Tray Service, but after printing something it does appear with the green check mark indicating it is the primary printer. It does show up in the list of printers available in Ubuntu, and I can print to it from any of the applications. But none of the applications which could scan before can find the 6700 any more, including XSane.

The scanner itself works - I can scan to a memory stick just fine, but can't use it with LibreOffice, Gimp, or XSane (the programs I use the scanner with most often).

I should mention several things that I wondered about:

1) The aforementioned "odd" behavior of the System Tray Service. When I first boot, the drop down menu seems like what I would expect. After using any of the devices though, it adds additional duplicate menu entries for "HP Device Manager ..." and "Settings" - the latter with no icon. Only one of each menu entry works, but clearly something is hincky here. But it's been that way for well over a year, so I don't know if it means anything.

2) I already had two HP devices installed (a 4700 and a K5400), but both are turned off. The result of this, for instance, is that when XSane scans for devices, it reports that the 4700 is missing, but it never discovers the 6700. Just last week, BOTH of these were turned on at the same time, and XSane discovered both and offered me a choice of which to read from.

3) When I go to the CUPS maintenance page the 6700 shows up, but as far as I understand, that has nothing to do with scanning.

I've attempted rebooting, disconnecting the USB cable and reconnecting it, powering the device itself off and on - none of this seems to "fix" the issue. When I first connected the device a month ago I was very pleasantly surprised to find that Ubuntu/CUPS/HPLIP/whatever automatically and immediately set up the 6700, and everything worked right out of the box. But I'm now afraid to completely remove the printer and start over again because I'm betting there is some file that will remain behind to screw it up when I reinstall it.

I've run the hp-check -t as your help site suggests, and found some interesting entries, such as "error: Unsupported model: Officejet_6700."

Where do I go from here? Any advice would be appreciated.

Output of hp-check -t is shown below.

foberle@ubu-Desktop:~$ hp-check -t

HP Linux Imaging and Printing System (ver. 3.13.11)
Dependency/Version Check Utility ver. 14.3

Copyright (c) 2011-14 Hewlett-Packard 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.

Note: hp-check can be run in three modes:
1. Compile-time check mode (-c or --compile): Use this mode before compiling the HPLIP supplied tarball (.tar.gz or
.run) to determine if the proper dependencies are installed to successfully compile HPLIP.
2. Run-time check mode (-r or --run): Use this mode to determine if a distro supplied package (.deb, .rpm, etc) or an
already built HPLIP supplied tarball has the proper dependencies installed to successfully run.
3. Both compile- and run-time check mode (-b or --both) (Default): This mode will check both of the above cases (both
compile- and run-time dependencies).

Saving output in log file: hp-check.log

Initializing. Please wait...

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

Basic system information:
Linux ubu-Desktop 3.2.0-58-generic #88-Ubuntu SMP Tue Dec 3 17:37:58 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

Distribution:
ubuntu 12.04

Checking Python version...
OK, version 2.7.3 installed

Checking PyQt 4.x version...
OK, version 4.9.1 installed.

Checking for CUPS...
Status: scheduler is running
Version: 1.5.3
error_log is set to level: warn

Checking for dbus/python-dbus...
dbus daemon is running.
python-dbus version: 1.0.0

------------------------------------
| COMPILE AND RUNTIME DEPENDENCIES |
------------------------------------

note: To check for compile-time only dependencies, re-run hp-check with the -c parameter (ie, hp-check -c).
note: To check for run-time only dependencies, re-run hp-check with the -r parameter (ie, hp-check -r).

Checking for dependency: CUPS - Common Unix Printing System...
OK, found.

Checking for dependency: CUPS devel- Common Unix Printing System development files...
OK, found.

Checking for dependency: CUPS image - CUPS image development files...
OK, found.

Checking for dependency: DBus - Message bus system...
OK, found.

Checking for dependency: gcc - GNU Project C and C++ Compiler...
OK, found.

Checking for dependency: GhostScript - PostScript and PDF language interpreter and previewer...
OK, found.

Checking for dependency: libcrypto - OpenSSL cryptographic library...
OK, found.

Checking for dependency: libjpeg - JPEG library...
OK, found.

Checking for dependency: libnetsnmp-devel - SNMP networking library development files...
OK, found.

Checking for dependency: libpthread - POSIX threads library...
OK, found.

Checking for dependency: libtool - Library building support services...
OK, found.

Checking for dependency: libusb - USB library...
OK, found.

Checking for dependency: make - GNU make utility to maintain groups of programs...
OK, found.

Checking for dependency: PIL - Python Imaging Library (required for commandline scanning with hp-scan)...
OK, found.

Checking for dependency: PolicyKit - Administrative policy framework...
OK, found.

Checking for dependency: PyQt 4 DBus - DBus Support for PyQt4...
OK, found.

Checking for dependency: Python DBus - Python bindings for DBus...
OK, found.

Checking for dependency: Python devel - Python development files...
OK, found.

Checking for dependency: Python libnotify - Python bindings for the libnotify Desktop notifications...
OK, found.

Checking for dependency: Python XML libraries...
OK, found.

Checking for dependency: Python 2.3 or greater - Required for fax functionality...
OK, found.

Checking for dependency: Python 2.2 or greater - Python programming language...
OK, found.

Checking for dependency: Reportlab - PDF library for Python...
OK, found.

Checking for dependency: SANE - Scanning library...
OK, found.

Checking for dependency: SANE - Scanning library development files...
OK, found.

Checking for dependency: scanimage - Shell scanning program...
OK, found.

Checking for dependency: xsane - Graphical scanner frontend for SANE...
OK, found.

----------------------
| HPLIP INSTALLATION |
----------------------

Currently installed HPLIP version...
HPLIP 3.13.11 currently installed in '/usr/share/hplip'.

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

[hplip]
version=3.13.11

[dirs]
home=/usr/share/hplip
run=/var/run
ppd=/usr/share/ppd/hplip/HP
ppdbase=/usr/share/ppd/hplip
doc=/usr/share/doc/hplip-3.13.11
html=/usr/share/doc/hplip-3.13.11
icon=/usr/share/applications
cupsbackend=/usr/lib/cups/backend
cupsfilter=/usr/lib/cups/filter
drv=/usr/share/cups/drv/hp
bin=/usr/bin

# 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=3.13.11
restricted-build=no
ui-toolkit=qt4
qt3=no
qt4=yes
policy-kit=no
lite-build=no
udev-acl-rules=yes
udev_sysfs_rules=no
hpcups-only-build=no
hpijs-only-build=no

Current contents of '/var/lib/hp/hplip.state' file:
# hplip.state - HPLIP runtime persistent variables.

[plugin]
installed=0
eula=0

Current contents of '~/.hplip/hplip.conf' file:
[settings]
systray_visible = 0
systray_messages = 0

[last_used]
device_uri = "hp:/usb/Officejet_Pro_K5400?serial=MY6CQ1806V"
printer_name = Photosmart_C4700_WiFi
working_dir = .

[commands]
scan = /usr/bin/simple-scan %SANE_URI%

[refresh]
rate = 30
enable = false
type = 1

[polling]
enable = false
interval = 5
device_list =

[fax]
voice_phone =
email_address =

[installation]
date_time = 02/03/2014 09:07:35
version = 3.13.11

[upgrade]
notify_upgrade = true
last_upgraded_time = 1386082693
pending_upgrade_time = 0
latest_available_version = 3.13.11

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

No devices found.

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

Officejet_6700
--------------
Type: Printer
Device URI: hp:/usb/Officejet_6700?serial=CN37DBRH4P05RQ
PPD: /etc/cups/ppd/Officejet_6700.ppd
PPD Description: HP Officejet 6700, hpcups 3.13.11
Printer status: printer Officejet_6700 is idle. enabled since Mon 03 Feb 2014 08:05:29 AM CST
error: Unsupported model: Officejet_6700
error: Communication status: Failed

Officejet_6700_fax
------------------
Type: Fax
Device URI: hpfax:/usb/Officejet_6700?serial=CN37DBRH4P05RQ
PPD: /etc/cups/ppd/Officejet_6700_fax.ppd
PPD Description: HP Fax4 hpcups
Printer status: printer Officejet_6700_fax is idle. enabled since Mon 20 Jan 2014 03:58:27 PM CST
error: Unsupported model: Officejet_6700
error: Communication status: Failed

Officejet_Pro_K5400
-------------------
Type: Printer
Device URI: hp:/usb/Officejet_Pro_K5400?serial=MY6CQ1806V
PPD: /etc/cups/ppd/Officejet_Pro_K5400.ppd
PPD Description: HP Officejet Pro k5400, hpcups 3.12.2
Printer Unplugged or turned offet_Pro_K5400 disabled since Fri 27 Dec 2013 09:22:25 PM CST -
error: Unable to communicate with device (code=12): hp:/usb/Officejet_Pro_K5400?serial=MY6CQ1806V
error: Device not found
error: Communication status: Failed

Photosmart_C4700_WiFi
---------------------
Type: Printer
Device URI: hp:/net/Photosmart_C4700_series?ip=192.168.1.103
PPD: /etc/cups/ppd/Photosmart_C4700_WiFi.ppd
PPD Description: HP Photosmart c4700 Series, hpcups 3.12.2
Printer status: printer Photosmart_C4700_WiFi is idle. enabled since Mon 20 Jan 2014 03:41:57 PM CST
error: Unable to communicate with device (code=12): hp:/net/Photosmart_C4700_series?ip=192.168.1.103
error: unable to open channel
error: Communication status: Failed

----------------------
| SANE CONFIGURATION |
----------------------

'hpaio' in '/etc/sane.d/dll.conf'...
OK, found. SANE backend 'hpaio' is properly set up.

Checking output of 'scanimage -L'...
device `hpaio:/net/Photosmart_C4700_series?ip=192.168.1.103' is a Hewlett-Packard Photosmart_C4700_series all-in-one

---------------------
| PYTHON EXTENSIONS |
---------------------

Checking 'cupsext' CUPS extension...
OK, found.

Checking 'pcardext' Photocard extension...
OK, found.

Checking 'hpmudext' I/O extension...
OK, found.

Checking 'scanext' SANE scanning extension...
OK, found.

-----------------
| USB I/O SETUP |
-----------------

Checking for permissions of USB attached printers...

HP Device 0x5c12 at 001:002:
    Device URI: hp:/usb/Officejet_6700?serial=CN37DBRH4P05RQ
error: Unsupported model: Officejet_6700

---------------
| USER GROUPS |
---------------

foberle adm lp cdrom floppy sudo dip plugdev lpadmin sambashare vboxusers

User member of group 'lp'. Enables print/ scan/ fax.
User member of group 'lpadmin'.

-----------
| SUMMARY |
-----------

error: 4 errors and/or warnings.

Please refer to the installation instructions at:
http://hplip.sourceforge.net/install/index.html

Done.