I just had this same problem after installing via the internet method (ie. install Kubuntu, then download install scripts from repo). Immediately after the post_installation script, I rebooted and happily went through the AV wizard. However, then the system left me with a black screen with just a cursor - after waiting patiently for 30-40 mins I figured something was wrong.
I rebooted and had the same problem, so went into my /etc/X11/xorg.conf file and found all sorts of complex settings in the screen, monitor and device subsections. I edited to match a known good xorg.conf from my old mythbuntu build and rebooted. Immediately after the system loaded normally (and proceeded to regenerate the UI)
This worked for me - hopefully the same works for you