"Login as another user" button having very odd results

[Wise Melon]

I have had a rather odd experience with the "Login as another user" button on the login screen. At first if I pressed it the screen would go black (except for a little underscore in the top left hand corner which was not flashing like in the classic Terminal), and then after a while come back grey and tell me that another application was blocking the lock screen (or something like that), and the only way to get back to the login screen (this could be also the locked screen) would be to change to a tty and then back again (for instance change to tty1, and then do "CTRL + ALT + F7" to get back).

Then when I pressed it today it did something different, the screen went black as it did before, but nothing I did would bring the login screen back (it was a locked screen and it should ask me for my password before logging me in again). So I went and logged into tty1, and then I typed "exit" to get back out, and it switched to a black screen again, then I tried going to tty1 again, but still black, and I could not return anywhere, then I tried going to tty2, it was a brand new session (I don't know if the other session was still running or whether it had been killed), and it had logged me in there without prompting for a password, which if successfully reproduced (which I haven't been able to do so far as I restarted my machine after I couldn't get back to my old session, or even figure out if it existed still), would be quite a major security bug.

After my restart I tried to reproduce the bug which logged me in, however I was unable to, although there were a lot of errors in syslog. But now when I go and click on the "Login as another user" button on the login or locked screen, what happens is that the screen goes black, and then I have to switch to another tty, and then back again to not get a black screen, but after this when I click the "Login as another user" button I get the login screen, with the Gnome logo at the bottom, but nothing else other than the top bar, just grey...

I know that this report may not be that helpful, as I can't reproduce everything, and it is rather long with a lot of issues, however I thought it good to report all that has happened so far so at least you know that these issues are happening.

I am running Ubuntu 15.04, with Gnome Shell, and Gnome 3.14.

---

OS Information:

    No LSB modules are available.
    Distributor ID: Ubuntu
    Description: Ubuntu 15.04
    Release: 15.04
    Codename: vivid

Package Information:

- GDM:

    gdm:
      Installed: 3.14.1-0ubuntu3
      Candidate: 3.14.1-0ubuntu3
      Version table:
     *** 3.14.1-0ubuntu3 0
            500 http://gb.archive.ubuntu.com/ubuntu/ vivid/universe amd64 Packages
            100 /var/lib/dpkg/status

- Gnome Shell:

    gnome-shell:
      Installed: 3.14.4-0ubuntu1
      Candidate: 3.14.4-0ubuntu1
      Version table:
     *** 3.14.4-0ubuntu1 0
            500 http://gb.archive.ubuntu.com/ubuntu/ vivid/universe amd64 Packages
            100 /var/lib/dpkg/status

[Daniel van Vugt (vanvugt)]

Thank you for reporting this bug to Ubuntu.
Ubuntu 15.04 (vivid) reached end-of-life on February 4, 2016.

See this document for currently supported Ubuntu releases:
https://wiki.ubuntu.com/Releases

We appreciate that this bug may be old and you might not be interested in discussing it any more. But if you are then please upgrade to the latest Ubuntu version and re-test.

[Launchpad Janitor (janitor)]

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

[Launchpad Janitor (janitor)]

[Expired for Ubuntu GNOME because there has been no activity for 60 days.]