Cannot login ubuntu after upgrade

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:

-----------------------------------
sudo nano /var/log/lightdm/lightdm.log
---------------------------------------
I had a suspicion on these lines:
 ..................
    [+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]:

---------------------------------------
[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
---------------------------------------

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 ^_^