hp-doctro fails due to missing cupsext

[JPi]

The following command

$ hp-doctor
error: cupsext not present in the system. Please re-install HPLIP.

fails.

I faced this by trying to make the scanner of Hewlett-Packard HP LaserJet M1120 MFP work in Ubuntu 19.10. The printer works but the scanner doesn't. I get the error

$ hp-scan

HP Linux Imaging and Printing System (ver. 3.19.6)
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.

-----------------
| SELECT DEVICE |
-----------------

  Num Scan device URI
  -------- -----------------------------------------------
  0 hpljm1005:libusb:007:007
  1 hpaio:/usb/HP_LaserJet_M1120_MFP?serial=MF30DE2

Enter number 0...1 for device (q=quit) ?1
warning: No destinations specified. Adding 'file' destination by default.
Using device hpaio:/usb/HP_LaserJet_M1120_MFP?serial=MF30DE2
Opening connection to device...
error: SANE: Error during device I/O (code=9)

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: hplip 3.19.6+dfsg0-1ubuntu1
ProcVersionSignature: Ubuntu 5.3.0-40.32-generic 5.3.18
Uname: Linux 5.3.0-40-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu8.4
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Sun Feb 23 13:22:20 2020
InstallationDate: Installed on 2020-02-06 (16 days ago)
InstallationMedia: Ubuntu 19.10 "Eoan Ermine" - Release amd64 (20191017)
Lpstat: device for HP-LaserJet-M1120-MFP: hp:/usb/HP_LaserJet_M1120_MFP?serial=MF30DE2
MachineType: System manufacturer System Product Name
Papersize: a4
PpdFiles: Error: command ['fgrep', '-H', '*NickName', '/etc/cups/ppd/HP-LaserJet-M1120-MFP.ppd'] failed with exit code 2: grep: /etc/cups/ppd/HP-LaserJet-M1120-MFP.ppd: Permission denied
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.3.0-40-generic root=UUID=49f08ac8-011f-45b7-b076-14e0c1484b76 ro quiet splash vt.handoff=7
SourcePackage: hplip
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 11/19/2019
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 1405
dmi.board.asset.tag: Default string
dmi.board.name: TUF GAMING X570-PLUS (WI-FI)
dmi.board.vendor: ASUSTeK COMPUTER INC.
dmi.board.version: Rev X.0x
dmi.chassis.asset.tag: Default string
dmi.chassis.type: 3
dmi.chassis.vendor: Default string
dmi.chassis.version: Default string
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr1405:bd11/19/2019:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKCOMPUTERINC.:rnTUFGAMINGX570-PLUS(WI-FI):rvrRevX.0x:cvnDefaultstring:ct3:cvrDefaultstring:
dmi.product.family: To be filled by O.E.M.
dmi.product.name: System Product Name
dmi.product.sku: SKU
dmi.product.version: System Version
dmi.sys.vendor: System manufacturer