HP print-to-fax always "on hold"

[David Kreindler]

I just replaced my HP1536dnf with a HP M479fdw. I'm running Ubuntu 20.04, with linux kernel version 5.8.0-44-generic. Ever since the change in printers, printing to the fax printer no longer works: the job shows up in the print queue, marked as 'held', and stays like that. Of note, the size of the job is listed as 0k, regardless of file size. Printing directly from the M479 works perfectly.

Reinstalling the queue using hp-setup -r and then hp-setup does not change this behaviour.
Upgrading hplip (from version 3.19* to 3.21.2 does not change this behaviour, either.

Help, please!

Output from hp-check:

===cut here====

HP Linux Imaging and Printing System (ver. 3.21.2)
Dependency/Version Check Utility ver. 15.1

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.

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).

Check types:
a. EXTERNALDEP - External Dependencies
b. GENERALDEP - General Dependencies (required both at compile and run time)
c. COMPILEDEP - Compile time Dependencies
d. [All are run-time checks]
PYEXT SCANCONF QUEUES PERMISSION

Status Types:
    OK
    MISSING - Missing Dependency or Permission or Plug-in
    INCOMPAT - Incompatible dependency-version or Plugin-version

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

 Kernel: 5.8.0-44-generic #50~20.04.1-Ubuntu SMP Wed Feb 10 21:07:30 UTC 2021 GNU/Linux
 Host: BlackBox
 Proc: 5.8.0-44-generic #50~20.04.1-Ubuntu SMP Wed Feb 10 21:07:30 UTC 2021 GNU/Linux
 Distribution: 12 20.04
 Bitness: 64 bit

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

HPLIP-Version: HPLIP 3.21.2
HPLIP-Home: /usr/share/hplip
HPLIP-Installation: Auto installation is supported for ubuntu distro 20.04 version

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

[hplip]
version=3.21.2

[dirs]
home=/usr/share/hplip
run=/var/run
ppd=/usr/share/ppd/hplip/HP
ppdbase=/usr/share/ppd/hplip
doc=/usr/share/doc/hplip-3.21.2
html=/usr/share/doc/hplip-3.21.2
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=3.21.2
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:
[plugin]
installed = 1
eula = 1
version = 3.21.2

Current contents of '~/.hplip/hplip.conf' file:
[commands]
scan = /usr/bin/xsane -V %SANE_URI%

[fax]
email_address =
voice_phone =

[installation]
date_time = 03/02/21 13:59:03
version = 3.21.2

[last_used]
device_uri = "hpfax:/net/HP_Color_LaserJet_Pro_M478f-9f?ip=192.168.209.46"
printer_name =
working_dir = .

[polling]
device_list =
enable = false
interval = 5

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

[settings]
systray_messages = 0
systray_visible = 0

[upgrade]
last_upgraded_time = 1614709719
latest_available_version = 3.17.10
notify_upgrade = true
pending_upgrade_time = 0

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

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

 cups CUPS - Common Unix Printing System REQUIRED 1.1 2.3.1 OK 'CUPS Scheduler is running'
 gs GhostScript - PostScript and PDF language interpreter and previewer REQUIRED 7.05 9.50 OK -
 xsane xsane - Graphical scanner frontend for SANE OPTIONAL 0.9 0.999 OK -
 scanimage scanimage - Shell scanning program OPTIONAL 1.0 1.0.29 OK -
 dbus DBus - Message bus system REQUIRED - 1.12.16 OK -
 policykit PolicyKit - Administrative policy framework OPTIONAL - 0.105 OK -
 network network -wget OPTIONAL - 1.20.3 OK -
 avahi-utils avahi-utils OPTIONAL - 0.7 OK -

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

 libjpeg libjpeg - JPEG library REQUIRED - - OK -
 cups-devel CUPS devel- Common Unix Printing System development files REQUIRED - 2.3.1 OK -
 cups-image CUPS image - CUPS image development files REQUIRED - 2.3.1 OK -
 libpthread libpthread - POSIX threads library REQUIRED - b'2.31' OK -
 libusb libusb - USB library REQUIRED - 1.0 OK -
 sane SANE - Scanning library REQUIRED - - OK -
 sane-devel SANE - Scanning library development files REQUIRED - - OK -
 libavahi-dev libavahi-dev REQUIRED - - OK -
 libnetsnmp-devel libnetsnmp-devel - SNMP networking library development files REQUIRED 5.0.9 5.8 OK -
 libcrypto libcrypto - OpenSSL cryptographic library REQUIRED - 1.1.1 OK -
 python3X Python 2.2 or greater - Python programming language REQUIRED 2.2 3.8.5 OK -
 python3-notify2 Python libnotify - Python bindings for the libnotify Desktop notifications OPTIONAL - - OK -
 python3-pyqt5-dbus PyQt 5 DBus - DBus Support for PyQt5 OPTIONAL 5.0 5.14.1 OK -
 python3-pyqt5 PyQt 5- Qt interface for Python (for Qt version 4.x) REQUIRED 5.0 5.14.1 OK -
 python3-dbus Python DBus - Python bindings for DBus REQUIRED 0.80.0 1.2.16 OK -
 python3-xml Python XML libraries REQUIRED - 2.2.9 OK -
 python3-devel Python devel - Python development files REQUIRED 2.2 3.8.5 OK -
 python3-pil PIL - Python Imaging Library (required for commandline scanning with hp-scan) OPTIONAL - 7.0.0 OK -
 python3-reportlab Reportlab - PDF library for Python OPTIONAL 2.0 3.5.34 OK -

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

 libtool libtool - Library building support services REQUIRED - 2.4.6 OK -
 gcc gcc - GNU Project C and C++ Compiler REQUIRED - 9.3.0 OK -
 make make - GNU make utility to maintain groups of programs REQUIRED 3.0 4.2.1 OK -

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

 cupsext CUPS-Extension REQUIRED - 3.21.2 OK -
 hpmudext IO-Extension REQUIRED - 3.21.2 OK -

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

 hpaio HPLIP-SANE-Backend REQUIRED - 3.21.2 OK 'hpaio found in /etc/sane.d/dll.conf'
 scanext Scan-SANE-Extension REQUIRED - 3.21.2 OK -

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

device `hpaio:/net/HP_Color_LaserJet_Pro_M478f-9f?ip=192.168.209.46' is a Hewlett-Packard HP_Color_LaserJet_Pro_M478f-9f all-in-one
device `pixma:04A91800' is a CANON Canon PIXMA TS8000 Series multi-function peripheral
device `escl:https://192.168.209.46:443' is a ESCL HP Color LaserJet M479f SSL flatbed scanner
device `escl:https://192.168.209.12:443' is a ESCL Canon TS8000 series SSL flatbed scanner
device `escl:http://192.168.209.46:8080' is a ESCL HP Color LaserJet M479f flatbed scanner
device `escl:http://192.168.209.12:80' is a ESCL Canon TS8000 series flatbed scanner
device `hpaio:/net/HP_Color_LaserJet_Pro_M478f-9f?ip=192.168.209.46' is a Hewlett-Packard HP_Color_LaserJet_Pro_M478f-9f all-in-one
device `pixma:04A91800' is a CANON Canon PIXMA TS8000 Series multi-function peripheral
device `escl:https://192.168.209.46:443' is a ESCL HP Color LaserJet M479f SSL flatbed scanner
device `escl:https://192.168.209.12:443' is a ESCL Canon TS8000 series SSL flatbed scanner
device `escl:http://192.168.209.46:8080' is a ESCL HP Color LaserJet M479f flatbed scanner
device `escl:http://192.168.209.12:80' is a ESCL Canon TS8000 series flatbed scanner

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

No devices found.

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

HP_M479
-------
Type: Printer
Device URI: hp:/net/HP_Color_LaserJet_Pro_M478f-9f?ip=192.168.209.46
PPD: /etc/cups/ppd/HP_M479.ppd
PPD Description:
Printer status: printer HP_M479 is idle. enabled since Tue 02 Mar 2021 01:45:34 PM
Communication status: Good

HP_M479_fax
-----------
Type: Fax
Device URI: hpfax:/net/HP_Color_LaserJet_Pro_M478f-9f?ip=192.168.209.46
PPD: /etc/cups/ppd/HP_M479_fax.ppd
PPD Description:
Printer status: printer HP_M479_fax is idle. enabled since Tue 02 Mar 2021 01:54:31 PM
Communication status: Good

TS8020
------
Type: Unknown
Device URI: cnijbe2://Canon/?port=usb&serial=313817
PPD: /etc/cups/ppd/TS8020.ppd
PPD Description:
Printer status: printer TS8020 is idle. enabled since Wed 06 Jan 2021 09:34:02 AM

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

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

Missing Required Dependencies
-----------------------------
None

Missing Optional Dependencies
-----------------------------
None

Total Errors: 0
Total Warnings: 1

Done.