Xorg fails, no gui: systemd issue?

[John -]

Since I last (3 December) upgraded the software (sid)  on my old Thinkpad, my 
gui fails to come up. The last line of /var/log/Xorg.0.log reads:
(EE) systemd-login: failed to take device /dev/dri/card0: Message recipient 
disconnected from message bus without replying
I've  been trying for weeks to fix it, including tracking down all suggests 
from googling the error message, without success. Can anyone help me figure out 
what the problem is?
Thanks.