Did you test : 3.28.2-0ubuntu1.1 ?
I thought that is The version having The fix


ke 30. toukokuuta 2018 klo 18.51 KenWilson <email address hidden>
kirjoitti:

> All,
> My testing indicates gdm3 3.28.0-0ubuntu1.1 fixes the incorrect password
> issue; however, it creates a new issue. As such I don’t think this update
> should be released.
>
> I installed gdm3 3.28.0-0ubuntu1.1 and rebooted my system (Ubuntu 18.04
> LTS). Making sure I typed the password correct I was able to login to
> each account on the system. I then logged out and performed the
> following steps to ensure the password issue was resolved.
>
> For two accounts I performed the following steps three times for each.
> Each time succeeded.
>
>     1. I typed a bad password and pressed enter. I did this two times.
> Each time I was returned to the login screen and received a message the
> login didn’t work.
>     2. I then typed a correct password and successfully logged in.
>     3. I then locked the screen and performed steps one and two and each
> time it worked as it should.
>     4. I then logged out of the account and performed steps one and two
> and each time it worked as it should.
> Based on this testing I would say recovering from a bad password is fixed.
>
> The next test I performed was to login into one account and then switch
> to the other account. I was able to successfully switch to the other
> account. I then tried to switch back to the first account but I was
> never able to successfully switch back. I tried it at least six times
> and in no case did it work. It did not matter which account I logged
> into first I was never able to switch to the other then switch back. In
> each case when I switched the second time the system hung and I could
> not find a way to recover. I tried CTRL+ALT+F2, CTRL+ALT+F1, CTRL+ALT+F2
> but there was still unable to login. I was not able to login to any
> account even when being extremely careful the password was correct.
>
> I then reinstalled gdm3 3.28.0-0ubuntu1 and tried switching between
> accounts. As long as I ensure I type the password correct I can switch
> between accounts multiple times without issue. If I type an incorrect
> password I can do CTR+ALT+F2, CTRL+ALT+F1, CTRL+ALT+F2 to recover.
>
> With package gdm3 3.28.0-0ubuntu1.1 the only way I was able to regain
> control (after trying to switch accounts a second time) was with a
> system reboot. To me, having to reboot is a catastrophic failure and
> therefor this package should not be released.
>
> --
> You received this bug notification because you are subscribed to a
> duplicate bug report (1768619).
> https://bugs.launchpad.net/bugs/1766137
>
> Title:
>   [regression] Password accepted but login fails (blank purple screen
>   and mouse pointer only)
>
> Status in gdm:
>   Fix Released
> Status in Release Notes for Ubuntu:
>   Fix Released
> Status in gdm3 package in Ubuntu:
>   Fix Released
> Status in gnome-shell package in Ubuntu:
>   Invalid
> Status in gdm3 source package in Bionic:
>   Fix Committed
> Status in gnome-shell source package in Bionic:
>   Invalid
>
> Bug description:
>   [ Description ]
>
>   Due to a refcounting bug, a GDBusConnection was getting disposed when
>   it was still required. The symptom of this was that you couldn't log
>   in on the second attempt if you'd got your password wrong on the first
>   attempt. All you'd see is a blank purple screen and mouse pointer
>   only.
>
>   [ Test case ]
>
>   1. Boot to GDM
>   2. Click your username
>   3. Type the wrong password a couple of times, pressing enter after each
> time
>   4. Type the right password
>
>   If the bug is happening, after 4. the system hangs at a blank screen
>   with the mouse cursor. If you then switch to a VT or otherwise connect
>   to the machine, you can examine the journal and you'll see a
>   G_IS_DBUS_CONNECTION failure.
>
>   [ Fix ]
>
>   Marco and I worked upstream on this fix. We found out that there was a
>   problem like this-
>
>   The GdmClient has a shared GDBusConnection for its operations. The
>   first time  someone calls for it, it is created and stored in the
>   object's private structure. Subsequent calls return *a new reference*
>   to this same object. It turned out that the asynchronous method to get
>   the connection was accidentally unreferencing its object before giving
>   it to the caller if it was returning an already-existing connection.
>
>   For this to work properly, we need to nullify the pointer we stored
>   when the connection goes away, so we know when to make a new one.
>   There were some cases where we didn't add the weak references required
>   to do that. Those are also fixed.
>
>   [ Regression potential ]
>
>   Now we share connections more than we did before. We also more
>   carefully track when to clear our object. If we got this wrong, we
>   might end up leaking the connection or dropping it in even more cases.
>
>   [ Original report ]
>
>   WORKAROUND: After typing an incorrect password, click Cancel, then
>   click your name, then enter your password again.
>
>   ---
>
>   Trying to log into my session (Gnome, Xorg), if I enter the wrong
>   password before entering it correctly, the session doesn't load and I
>   get a purple screen, a mouse cursor, and an invisible but clickable
>   menu in the top right. If I enter it correctly the first time, there
>   is no problem.
>
>   I've replicated this from a fresh boot, after logging out and after
>   'sudo service gdm restart' from the Ctrl-Alt-F4 console.
>
>   This is a fresh install, and didn't occur when I was using a previous
>   install of 18.04 (until Friday).
>
>   ProblemType: Bug
>   DistroRelease: Ubuntu 18.04
>   Package: gdm3 3.28.0-0ubuntu1
>   ProcVersionSignature: Ubuntu 4.15.0-15.16-generic 4.15.15
>   Uname: Linux 4.15.0-15-generic x86_64
>   ApportVersion: 2.20.9-0ubuntu6
>   Architecture: amd64
>   CurrentDesktop: ubuntu:GNOME
>   Date: Sun Apr 22 20:31:13 2018
>   InstallationDate: Installed on 2018-04-22 (0 days ago)
>   InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64
> (20180421.1)
>   SourcePackage: gdm3
>   UpgradeStatus: No upgrade log present (probably fresh install)
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/gdm/+bug/1766137/+subscriptions
>