Hi, I got one more hint for this bug. I installed KDM and it works, but only the xfce session was able to start. I also got installed Gnome and Enlightenment. I also tried to use Lightdm and it starts and shows the login prompt but it doesn't let me start any of the sessions named before.
I'll use KDM for now because I got some things I want to do in this virtual machine. But I'm still interest in using GDM so I'll keep trying to solve the problem by myself. Thanks anyway!