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 How to debug Wayland problems - Fedora Project Wiki 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.