Do you have any disabled monitors plugged in? This sounds like bug
1760849
** Package changed: xorg (Ubuntu) => gnome-shell (Ubuntu)
** Changed in: gnome-shell (Ubuntu)
Status: New => Incomplete
--
You received this bug notification because you are a member of Ubuntu-X,
which is subscribed to xorg in Ubuntu.
https://bugs.launchpad.net/bugs/2042823
Title:
Login blank background (gray) after update
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/2042823/+subscriptions
___
Mailing list: https://launchpad.net/~ubuntu-x-swat
Post to : ubuntu-x-swat@lists.launchpad.net
Unsubscribe : https://launchpad.net/~ubuntu-x-swat
More help : https://help.launchpad.net/ListHelp