LightDM freezes at first startup

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http://askubuntu.com – Nearly every time I bootup my Ubuntu I come to the login screen. But instead of showing me the wallpaper from selected account, I just see the default purple background with those white dots on it. Anyway, at this screen I'm neither able to login (the text field does not accept input) nor to shut down, because I am unable to click the icon in the upper left (although I can move the mouse). To work arond this problem, I switch to a TTY using STRG+ALT+F2, login and reboot using sudo reboot. After a restart, I can log in normally, the login screen will appear with my account's wallpaper. Also in (HowTos)