blank screen (no backlight) after login on both xubuntu and ubuntu desktops, Optimus laptop

[Tim Richardson]

I have a Thinkpad W520. In Optimus mode, lightdm presents a blank screen after login. Virtual terminals work.
On the same laptop, gdm3 allows me to log in to ubuntu desktop.
In pure Nvidia mode, login works.

The bug report tool told me that log files will be attached.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: lightdm 1.26.0-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-15.16-generic 4.15.15
Uname: Linux 4.15.0-15-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu4
Architecture: amd64
Date: Sun Apr 15 19:55:08 2018
InstallationDate: Installed on 2018-02-25 (49 days ago)
InstallationMedia: Xubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180225)
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)

[Tim Richardson (tim-richardson)]

attachment is generated by the nvidia-bug-reporting tool, which collects many logs