Ubuntu – Cannot Log In to Gnome 3.16 Dekstop

I have installed Gnome 3.16 on my Ubuntu 15.04 (previously updated from 14.04) via:

sudo add-apt-repository ppa:gnome3-team/gnome3
sudo add-apt-repository ppa:gnome3-team/gnome3-staging
sudo apt-get install gnome

However, I am unable to log into the Gnome desktop:

• If I select gdm as my default desktop manager, my system boots to a black screen only showing the cursor. I can switch into tty, but no other X component loads.
• If I leave lightdm selected, I am able to log in to Unity, but if I choose either Gnome or Gnome Classic I get a black screen for a couple of seconds after typing my password, then I'm back at the login screen. Gnome Flashback (both Compiz and Metacity options) works, but they are obviously not what I'm interested in using.

I've tried different suggestions from similar, but older, questions, including:

• Removing any nvidia-* packages
• Removing my .Xauthority file
• Manually stopping gdm then running startx

None of these brought any improvement. The only potentially useful log info I can find is in the rather peculiarly-named /var/log/gdm/(null)-greeter.log:

(WW) xf86CloseConsole: KDSETMODE failed: Input/output error
(WW) xf86CloseConsole: VT_GETMODE failed: Input/output error
(EE) 
Fatal server error:
(EE) xf86CloseConsole: VT_ACTIVATE failed: Input/output error
(EE) 
(EE) 
Please consult the The X.Org Foundation support 
         at http://wiki.x.org
 for help. 
(EE) Please also check the log file at "/var/log/Xorg.0.log" for additional information.
(EE) 
(EE) 
(EE) Backtrace:
(EE) 0: /usr/bin/X (xorg_backtrace+0x56) [0x7f13a8d92556]
(EE) 1: /usr/bin/X (0x7f13a8bdf000+0x1b7749) [0x7f13a8d96749]
(EE) 2: /lib/x86_64-linux-gnu/libc.so.6 (0x7f13a68a5000+0x352f0) [0x7f13a68da2f0]
(EE) 3: ?? [0x7f13ab7f08c0]
(EE) 
(EE) Segmentation fault at address 0x7f13ab7f08c0
(EE) 
FatalError re-entered, aborting
(EE) Caught signal 11 (Segmentation fault). Server aborting

There is no error message in /var/log/Xorg.0.log.

Has anyone seen this before and do they have any suggestions?

Thank you.