Insanely, the boot before, and the boot after, were perfectly smooth and successful, and stayed on for hours. This single startup required a force restart after hanging on the purple that comes after typing password and hitting enter on login, and just before normal gnome desktop appears.
I'm not sure it's lightdm, but I read that lightdm still runs the login screen, and was responsible for previous problems at this stage in the bootup process.
ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: lightdm (not installed)
ProcVersionSignature: Ubuntu 4.15.0-22.24-generic 4.15.17
Uname: Linux 4.15.0-22-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.1
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Wed Jun 6 12:50:56 2018
InstallationDate: Installed on 2018-04-26 (41 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180228)
ProcEnviron:
TERM=xterm-256color
PATH=(custom, no user)
XDG_RUNTIME_DIR=<set>
LANG=en_US.UTF-8
SHELL=/bin/bash
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)
Thank you for taking the time to report this bug and helping to make Ubuntu better. This particular bug has already been reported and is a duplicate of bug 1766137, so it is being marked as such. Please look at the other bug report to see if there is any missing information that you can provide, or to see if there is a workaround for the bug. Additionally, any further discussion regarding the bug should occur in the other report. Feel free to continue to report any other bugs you may find.