Black screen, UI lock up, when starting Xserver,

[John Talbut]

I am trying to reinstall Debian Buster/Testing on a Samsung NC110
netbook after installing a new hard disk. I have got as far as
installing the base system and the packages for xfce. Now when I try to
boot I get past the grub screen, put in the password for the encrypted
disk, the boot appears to proceed normally until the desktop manager
login screen should appear then the screen goes black and the keyboard
becomes unresponsive.

I get the same symptoms using a different desktop manager and I get the
same lock up if I use startx.

The system has worked fine for a long time up to last July (i.e. 2018).

The system is not crashing, lightdm goes into this loop:

systemd[1]: Starting Light Display Manager
lightdm[2822]: Could not enumerate user data directory
/var/lib/lightdm/data: Error opening directory '/var/lib/lightdm/data':
No such file or directory
systemd[1]: Started Light Display Manager
systemd[1]: lightdm.service: Main process exited, code=exited,
status=1/FAILURE
systemd[1]: lightdm.service: Failed with result 'exit-code'.
systemd[1]: lightdm.service: Service RestartSec=100ms expired,
scheduling restart
systemd[1]: lightdm.service: Scheduled restart job, restart counter is
at  <>.
systemd[1]: Stopped Light Display Manager

The only errors that show in Xorg.0.log are:

[  420,876](EE) modeset(0): [DRI2] No driver mapping found for PCI
device 0x8086 / 0x0be1
[  42.877] (EE) modeset(0): Failed to initialize the DRI2 extension

I.e. the graphic controller is a VGA compatible controller: Intel
Corporation Atom Processor D2xxx/N2xxx Integrated Graphics Controller
(rev 09)

Questions, for a start:
1. What is supposed to provide the mapping for PCI device 0x8086 / 0x0be1?

2. Is this error likely to be causing the UI to lock up?

John