Switching to the NVIDIA proprietary graphics driver led to a crash. I did not have a live disk, so I had to do a full re-install.
Couple of tweaks. First of all, there is a more rigorous fix for the NVIDIA driver boot issue here. At least there is now a Grub menu so that debugging is possible.
I have not yet dared to use the NVIDIA driver again. But in the start-up logs there is still an error related to the open source driver which seems to slow down the boot process.
To see boot errors:
journalctl -b | grep error
One of the things I try is this to solve the NXDOMAIN error that I saw in the boot log:
sudo rm -r /etc/resolv.conf sudo ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
Also followed this tip and re-installed Gnome:
sudo apt-get install --reinstall systemd gnome-settings-daemon gnome-settings-daemon-common
And enabled canonical-livepatch with a new token, as described here.
sudo snap install canonical-livepatch sudo canonical-livepatch enable [#yourverylongtoken#]