Debian Stretch : X server won't start after update

[Bernard <bdebreil@free.fr>]

Hi to Everyone,

I am writing from my laptop on Ubuntu, but the problem I have is with my 
Desktop running Debian Stretch.
Having unfortunately Okayed a proposal for an update (which I rarely 
do…), upon restarting the system fails to launch X server.

My /var/log/messages are available at

http://bdebreil.free.fr/messages

starting line 7712 :

April 14 – 0858 – Failed to call gs_plugin_update on system-updates : 
waiting time max is over

Debian Stretch gnome session binary (1256) …. failed
       (msg repeated 5 times)
GLIB-Gobject-critical : gobject_unref_assertion 'G S OBJECT failed

…………………………………………….

Debian-Stretch gnome shell[1345] ; /build/ … has no handler with id '235895'
………………………………
line 7725
PolicyKit daemon disconnected from the bus
We are no longer a registered authentication agent………
……………………………………..

I've attempted to shutdown and restart several times, I tested the 
starting with 'Advanced Options' and then called 'startx', to no better 
result.

How can I get back to my old system prior to this failed update ?  What 
else can I do ?

Thanks in advance for your help in restoring my system

Bernard