This happens sometimes (~70% to happen) after I upgrdaded my ubuntu to 18.04. After entering user & password I see only blank purple screen without anything. I was trying to wait for 30 minutes, but it seemed like nothing was happening. This problem affects gnome-session and ubuntu-session for me.
I've also found that I'm not the only person who is affected.
ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gdm3 3.28.0-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CurrentDesktop: GNOME
Date: Tue May 8 01:51:42 2018
InstallationDate: Installed on 2017-11-11 (177 days ago)
InstallationMedia: Ubuntu 17.10 "Artful Aardvark" - Release amd64 (20171018)
ProcEnviron:
TERM=xterm-256color
PATH=(custom, no user)
XDG_RUNTIME_DIR=<set>
LANG=ru_RU.UTF-8
SHELL=/bin/bash
SourcePackage: gdm3
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.gdm3.custom.conf: 2018-03-02T21:56:59.591943
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.