samsung clp-680nd does not print duplex

My Samsung CLP-680ND printer is set up on a wired network. It works correctly from a Windows 10 laptop, printing duplex (double sided) pages. On Ubuntu 18.04 and 18.10 it does not print duplex although the option can be selected on the print dialogue. Otherwise printing is OK.

It used to work but I don't know which upgrade broke it.

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: cups 2.2.8-5ubuntu1.3
ProcVersionSignature: Ubuntu 4.18.0-20.21-generic 4.18.20
Uname: Linux 4.18.0-20-generic x86_64
ApportVersion: 2.20.10-0ubuntu13.3
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Thu May 30 22:47:02 2019
Lpstat:
 device for CLP-680: dnssd://Samsung%20CLP-680%20Series%20(SEC84251901BD4B)._ipp._tcp.local/?uuid=16a65700-007c-1000-bb49-84251901bd4b
 device for QL-500: usb://Brother/QL-500?serial=K9G909359
 device for Samsung_CLP_680_Series_SEC84251901BD4B_: ipp://SEC84251901BD4B.local:631/ipp/printer
MachineType: Dell Inc. OptiPlex 990
Papersize: a4
PpdFiles:
 Error: command ['fgrep', '-H', '*NickName', '/etc/cups/ppd/QL-500.ppd', '/etc/cups/ppd/Samsung_CLP_680_Series_SEC84251901BD4B_.ppd', '/etc/cups/ppd/CLP-680.ppd'] failed with exit code 2: grep: /etc/cups/ppd/QL-500.ppd: Permission denied
 grep: /etc/cups/ppd/Samsung_CLP_680_Series_SEC84251901BD4B_.ppd: Permission denied
 grep: /etc/cups/ppd/CLP-680.ppd: Permission denied
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.18.0-20-generic root=UUID=aa06cf80-1d26-4a3d-942b-452087493fe9 ro quiet splash vt.handoff=1
SourcePackage: cups
UpgradeStatus: Upgraded to cosmic on 2019-05-30 (0 days ago)
dmi.bios.date: 07/25/2011
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A06
dmi.board.name: 0VNP2H
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 3
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA06:bd07/25/2011:svnDellInc.:pnOptiPlex990:pvr01:rvnDellInc.:rn0VNP2H:rvrA00:cvnDellInc.:ct3:cvr:
dmi.product.name: OptiPlex 990
dmi.product.version: 01
dmi.sys.vendor: Dell Inc.