Ubuntu – Ubuntu 14.04 boots to black screen after upgrade from 13.10 – no display, problem seems to be X connected

I just spent a day trying to work around my newly upgraded Ubuntu 14.04 booting into a black screen without success and becoming seriously desperate.

What I did: Upgrade to 14.04 LTS from 13.10 via command line. Everything proceeded without problems.

The status now: GRUB shows. After I select any Ubuntu starting option I still have on my system, the Ubuntu start screen with the dots appears, changes into a black screen with a white cursor in the upper front corner. At this stage nothing will get me out of this besides a manual restart (Alt + F1 or F2 does not work, no keyboard reactions otherwise). This effect also happens with the Ubuntu 13.10 kernel stilled imaged on the system. Graphics failsafe modes won't boot at all. I can get to a text mode boot via selecting recovery mode and from there return to normal boot –> login to 14.04 works, but no display is recognized.

My System:

Thinkpad X1, dualboot with Windows 7. Intel HD 4000 graphics card.
There is no file /etc/X11/xorg/conf.

lspci -nnk | grep -iA3 vga Yields:

00:02.0 VGA compatible controller [0300]: Intel Corporation 3rd Gen Core processor Graphics Controller [8086:0166] (rev 09)

Subsystem:

Lenovo Device [17aa:21f9]
Kernel driver in use: i915
00:14.0 USB controller [0c03]:
Intel Corporation 7 Series/C210 Series Chipset Family USB xHCI Host Controller[8086:1e31] (rev 04)

/usr/lib/nux/unity_support_test -p yields:

No protocol specified
  Error: unable to open display

What I tried:
Many suggested solutions from other forum threads, including
Resetting unity, dpkg –reconfigure, several installations and re-installation of fglrx / nouveau / nvidia graphics drivers, reinstalling X, …….

Booting 14.04 from a live USB, which works great and enables me to write this post, but I don't manage to fix my local 14.04 installation