Locked out of my user account

[Mariano Jara Melagrani]

I can't login to the X session on my account. This is the user that's
been created when the system was installed, that is to say, this user
has sudo access.

I typed the right password and nothing happens, it stays at the
greeter screen, there's no error message or whatsoever. I know it's
the right password because I can login to the term console with <ctrl
+ alt + F1-6>.

At the console I've had quick a look at
/var/log/auth.log
/var/log/Xorg.0.log
/var/log/dmseg
but I couldn't find anything that stroke my attention plus I don't
know exactly what and where to look for.

There are 5 users in this system (2 with sudo access and 3 regular
users), plus the guest user. Out of the 5 user, at least 3 can login
without problems. For clarification purposes I'll put it like this:
User 1 (sudo access) --> This is the user that can't login to X
session, but can to console.
User 2 (sudo access) --> It can login, no problems here.
User 3 (regular user) --> It can login.
User 4 (regular user) --> It can login.
User 5 (regular user) --> The user is not here and I don't know their password.

This system is running Ubuntu 14.04 64 bit fully updated with Unity
(default) Compiz I believe, but it's not a fresh install, it has been
upgraded from 10.04 or maybe 10.10, not sure, I can't remember.

$ cat /proc/version_signature
Ubuntu 3.13.0-29.53-generic 3.13.11.2

Can someone point me in the right direction? Where should I start
looking for the culprit? Or what other info or log do you need me to
post? TIA

Mariano.