Touchpad stopped working after kernel update

[Rolf Kutz]

Synaptics touchpad stopped working when booting with kernel 4.15.0-72.81~1. When booting with the older Kernel 4.15.0-70.79~1 it works normally.

System information:
Description: Ubuntu 16.04.6 LTS
Release: 16.04

linux-image-4.15.0-72-generic:
  Installiert: 4.15.0-72.81~16.04.1
  Installationskandidat: 4.15.0-72.81~16.04.1
  Versionstabelle:
 *** 4.15.0-72.81~16.04.1 500
        500 http://de.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages
        100 /var/lib/dpkg/status

What is happening:
The touchpad is detected by the system, but evtest shows no output (https://wiki.ubuntu.com/DebuggingTouchpadDetection). Booting with an older Kernel shows normal behaviour.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.15.0-72-generic 4.15.0-72.81~16.04.1
ProcVersionSignature: Ubuntu 4.15.0-70.79~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-70-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.21
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Jan 1 20:27:43 2020
InstallationDate: Installed on 2016-11-20 (1136 days ago)
InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
SourcePackage: linux-signed-hwe
UpgradeStatus: No upgrade log present (probably fresh install)