Why can't I log into Lubuntu after the 13.04 update, even if I know the password?

After the 13.04 update the login screen started to appear, even though I chose to automatically login in the Lubuntu install options one week ago.

Even if my password and username is correct it won't login, it will just come back to the login screen.

When I press ALT-CTRL-F1 at the login screen I am able to login perfectly, but I have no GUI and I'm kind of lost on that field.

WHat is going?Is it just a bug? Can I downgrade?I tried several forums with different solutions but none of them worked.