Hi friends,
I'm building a custom live Ubuntu Gnome 3.12.2 based,
**Only** if I create a new user, set password and add him to sudo group,

setting by dpkg-reconfigure, GDM ad default greeter, obtain a black screen and going to the first console appear "authentication failure" message.

using dpkg-reconfigure and select the Lightdm as default greeter all work fine: the greeter show the login screen and the user is able to authenticate.

if I do not use any user, and set GDM ad default greeter, all work fine, but no login screen appear and the user goes directly to its desktop.

For build the iso I'm using the Ubuntu directive: https://help.ubuntu.com/community/LiveCDCustomization

Could you help me find a solution to this problem?

many thanks!