Bug#744152: unable to login with gdm3

[Giorgio Valocchi]

When i tried to login with gdm3 (version 3.8.4-7) the screen switch off 
and then display the message "authentication error".
If I try to login again it load forever.
I tried to upgrade to gdm3 version 3.8.4-8 (i'm able to login in tty1) 
but i have still the same problem.
So i installed kde and with kdm I'm able to login in gnome.
I don't have fglrx but mesa driver.