Sometime ago, I had a problem, ie cannot login to ubuntu 14.04, but now I have succeed to login ^_^. This problem also have happened to older Ubuntu version, where I get to know the solution. In my case there was a mismatch on desktop config from the user I login and the available desktop config. Here what i did: ([username] just as a placeholder)
First I check the lightdm.log:
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sudo nano /var/log/lightdm/lightdm.log
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I had a suspicion on these lines:
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[+27.41s] DEBUG: Session pid=1114: User [username] authorized
[+27.42s] DEBUG: Session pid=1114: Greeter requests session ubuntu
[+27.42s] DEBUG: Seat: Failed to find session configuration ubuntu
[+27.42s] DEBUG: Seat: Can't find session 'ubuntu'
..................
why did lightdm look for “ubuntu” session? This was because of [username] which i use its xsession is ubuntu. It was on /var/lib/AccountsService/users/[username]:
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[User]
Language=en_US
FormatsLocale=id_ID.UTF-8
XSession=ubuntu <===================== THIS ONE
Background=/home/[username]/Pictures/Wallpapers/1781785_10201241375132193_1896210726_o.jpg
SystemAccount=false
[InputSource0]
xkb=us
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Then I look at the xsession in folder /usr/share/xsessions. There was only gnome.desktop, no ubuntu.desktop. So that I changed the content of /var/lib/AccountsService/users/[username]:
from XSession=ubuntu to XSession=gnome
Problem solved ^_^