I had been using Debian Tesing with Openbox and I wanted to see how GNOME 3 in Debian was. So, I decided to install GNOME 3.
After the installation, instead of showing GNOME login screen, my system displayed a black screen with this error dialog: Failed to load session “gdm-fallback”.
“gdm-fallback”? Well, I thought it was a problem with my NVIDIA card. After a few hours trying to fix, I realized that I was wrong. The cause was logged right in the log file /var/log/gdm3/:0-greeter.log that a component of polkit-gnome-authentication-agent-1 was missing.
In the past, there was a desktop file for polkit-gnome-authentication-agent-1 in /etc/xdg/autostart/ to start polkit-gnome-authentication-agent-1 at boot. But now, the desktop file is not neccessary because gnome-session package handle this.
I have no idea why my gnome-session didn’t start PolicyKit agent, because in my virtual Debian Testing machine everything was fine. (I tested by installing GNOME 3 from a command line Debian).
I created polkit-gnome-authentication-agent-1.desktop file in /etc/xdg/autostart/ and now everything is fine. GNOME 3 is fast, beautiful and I have not faced any bug yet.
This is the content of my polkit-gnome-authentication-agent-1.desktop:
Name=PolicyKit Authentication Agent
Comment=PolicyKit Authentication Agent