Cannnot use printer because of some apparmor access right

[Richard]

My Epson printer disappeared from the list of registered printers.

I could not assign a suitable driver, even though a relevant package supplied by Epson was already installed (under /opt).
It turned out that the logs contained messages like this one:

audit: type=1400 audit(1602529987.370:8039): apparmor="DENIED" operation="open" profile="/usr/sbin/cupsd" name="/home/_opt/epson-inkjet-printer-stylus-photo-px810fw-series/ppds/" pid=29169 comm="cups-driverd" requested_mask="r" denied_mask="r" fsuid=7 ouid=0

As explained in https://wiki.ubuntu.com/DebuggingPrintingProblems#AppArmor_Protection_of_the_printing_system,
a workaround is to install apparmor-utils and run `aa-complain cupsd'.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: cups 2.3.1-9ubuntu1.1
ProcVersionSignature: Ubuntu 5.4.0-7642.46~1598628707~20.04~040157c~dev-generic 5.4.44
Uname: Linux 5.4.0-7642-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.9
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Mon Oct 12 23:30:38 2020
InstallationDate: Installed on 2020-08-14 (59 days ago)
InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
Lpstat: device for Epson-Stylus-Photo-PX710W: dnssd://EPSONF904BC._printer._tcp.local/
MachineType: System76, Inc. Pangolin Performance
Papersize: a4
PpdFiles: Error: command ['fgrep', '-H', '*NickName', '/etc/cups/ppd/Epson-Stylus-Photo-PX710W.ppd'] failed with exit code 2: grep: /etc/cups/ppd/Epson-Stylus-Photo-PX710W.ppd: Permission denied
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.4.0-7642-generic root=UUID=675a1866-9c2b-49b9-a2c5-c131e736d65c ro quiet splash acpi_os_name=Linux acpi_osi= vt.handoff=7
SourcePackage: cups
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 06/25/2012
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 4.6.5
dmi.board.asset.tag: Tag 12345
dmi.board.name: Pangolin Performance
dmi.board.vendor: System76, Inc.
dmi.board.version: panp9
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: No Enclosure
dmi.chassis.version: N/A
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr4.6.5:bd06/25/2012:svnSystem76,Inc.:pnPangolinPerformance:pvrpanp9:rvnSystem76,Inc.:rnPangolinPerformance:rvrpanp9:cvnNoEnclosure:ct10:cvrN/A:
dmi.product.family: Not Applicable
dmi.product.name: Pangolin Performance
dmi.product.sku: Not Applicable
dmi.product.version: panp9
dmi.sys.vendor: System76, Inc.