After a successful offline system upgrade of a Fedora 29 , GDM cannot boot into a GNOME Wayland nor into a GNOME via Xorg session. When trying GNOME classic, it displays a distorted desktop, showing only frameless windows of tray applications.
The system is fully accessible via the framebuffer console, autoconnects to WiFi and is else up and running.
From https://fedoraproject.org/wiki/How_to_debug_Wayland_problems we learn to check that journalctl -ab registers a core dump of gnome-shell, and also few errors with custom installed extensions right before.
mv ~/.local/share/gnome-shell/extensions ~/tmp/
systemctl restart gdm
helped to restore the Wayland desktop session.