I had the same (or very similar) problem some months ago, but after a while it disappeared. When existed I could use the system using upstart element of the Grub menu. After an October update of 16.04 it returned and produces the error report starting with
ExecuteablePath /usr/lib/xorg/Xorg Package xserver-xorg-core 2:1.18.4-0ubuntu0.8 ProblemType Crash Title Xorg crashed with SIGABRT in OsAbort()
I went through steps of UPDATE APRIL: there were no ~/.config/upstart/lowgfx.conf file, I deleted files of ~/.cache/ library (and left its sublibraries), there were no /var/crash/ files, I have added the terminal command sed -i 's/-lowgfx//' ~/.config/compiz-1/compizconfig/config, restarted the system with ubuntu menu.
There were no changes in the behaviour of the system (1)the restart stopped with ubuntu on the middle of the screen with 5 dots running below it, and press of Esc resulted black screen (2)upstart (or recovery + resume) works (3)no revoke from sleep
What to do?