Had some problems with Gnome failing to start and crashing with the weird sad face image and ‘Oh no, something has gone wrong’ error. If you check the error journal with ‘journalctl | grep gnome’ you may see some errors relating to PolKit. This can be checked against specifically with ‘journalctl | grep polkit’. If PolKit is throwing errors, installing polkit-gnome again. This seemed to repair paths, dependencies or whatever and after this Gnome booted successfully.