Computer won't suspend, DBUS reports that a suspend or shutdown operation already in progress

[A.Kromic]

After a fresh boot computer suspends fine, however further attempts to suspend result in failure with the above mentioned error message. When that happens, shutdown doesn't work either; shutdown process gets stuck at some point and I must forcibly turn off or reset the computer. Sometimes the shutdown almost completes, but THEN the computer enters suspend, after which I must wake it up to complete shutdown.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: dbus 1.12.2-1ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-32.35-generic 4.15.18
Uname: Linux 4.15.0-32-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CurrentDesktop: XFCE
Date: Fri Aug 24 10:39:30 2018
SourcePackage: dbus
UpgradeStatus: Upgraded to bionic on 2018-06-19 (65 days ago)

[Theo Linkspfeifer (lastonestanding)]

Please test with the 'nouveau' graphics driver and/or with light-locker disabled or replaced with another screen locker.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Launchpad Janitor (janitor)]

[Expired for dbus (Ubuntu) because there has been no activity for 60 days.]