I had a similar issue Ubuntu 12.04 login screen finite loop and keep end my session.
Can you check if you're using any PPA xorg-edgers packages like Version: 2:2.21.0+git20130204.9640640a-0ubuntu0ricotz~precise at xserver-xorg-video-intel?
$ apt-cache show xserver-xorg-video-intel | grep "Version"
Version: 2:2.21.0+git20130204.9640640a-0ubuntu0ricotz~precise
Version: 2:2.19.0-0ubuntu1~xup1
Version: 2:2.17.0-1ubuntu4.2
Version: 2:2.17.0-1ubuntu4
$ apt-cache show xserver-xorg-core | grep "Version"
Recently I encountered this problem with kubuntu 16.04 SDDM login screen hangs. The issue occurred after I installed kubuntu 16.04 on an Intel i7-7700k with Intel 630 HD graphics and then updated to the latest packages.
I was able to login using the workaround of using Ctrl+Alt+F1 to get to a text console and then Ctrl+Alt+F7 to get back to X and the logged in session.
I stumbled upon a fix to the login hang while I was working to get rid of a screen tearing problem. The fix involves changing the compositor rendering backend from XRender to OpenGL.
More specifically to address the hang on login and problem with screen
tearing I did the following:
FIX SCREEN TEARING AND LOGIN HANGING
A) Start K -> Settings -> System Settings
Under Hardware section
Double-click Display and Monitor
Choose Compositor on LHS
Enable compositor on startup: Checked # was checked
Rendering backend: OpenGL 3.1 # was XRender
OpenGL interface: GLX
Tearing prevention ("vsync"): Full screen repaints # was Automatic
Ignore message "Full screen repaints" can cause performance problems.
Click Apply
REFERENCES:
Bug 321589 - Automatic VSync strategy causes tearing
https://bugs.kde.org/show_bug.cgi?id=321589
Screen Tearing on Kubuntu 16.04 with Intel Drivers?
https://askubuntu.com/questions/764302/screen-tearing-on-kubuntu-16-04-with-intel-drivers
B) Set KWIN variables.
Edit /etc/environment and add the top and bottom line:
__GL_YIELD="USLEEP"
#KWIN_TRIPLE_BUFFER=0
KWIN_USE_BUFFER_AGE=0
REFERENCES:
https://community.kde.org/KWin/Environment_Variables
https://askubuntu.com/questions/764302/screen-tearing-on-kubuntu-16-04-with-intel-drivers/774788
Listed as also working with NVidia.
I hope the above steps help others get rid of the login hang and screen tearing problems.
Best Answer
I have the same problem and found out, that there is already a confirmed bug reported on launchpad for this problem: https://bugs.launchpad.net/lightdm/+bug/986967
I wrote a question for this problem on askubuntu myself with a bunch of log output ... 12.04: LightDM freezes while "Logging in..." - maybe due to deactivated login sound
maybe it's an authentication problem, because when I can login, this line in auth.log is present:
and when I have the problem, this line isn't there...
(I somehow can't comment so I wrote an answer... I hope that isn't rude or something)
edit: There is an unofficial bugfix posted in launchpad. I didn't try it yet.
edit: People in Launchpad report that the bugfix is working, and that the problem may be caused by deactivating the login sound with Ubuntu Tweak. Maybe reactivating the login sound might fix this problem too until a official update is available