journalctl -b -1 -e
There I ran an update from the GUI, which may have helped. However, there was a warning about an issue with a screensaver the first time that I loaded lightdm, and the crashes seemed to happen sometime after a clean boot. So I uninstalled lightdm, and installed gdm (which failed as installed already) and rebooted but got no GUI. Then I uninstalled and reinstalled gdm and everything seems finenow.
apt-get remove lightdm
apt-get remove gdm3