Blank screen instead of X session (xfce). How to fix?

[John]

This is a cross post from stackoverflow:
http://stackoverflow.com/questions/20813281/blank-screen-instead-of-x-session-xfce-how-to-fix

I've updated my mythbuntu box from 12.04 to 13.10 and updated mythtv
from 0.25 to 0.27. Now when I start the box I just get a blank screen.
xfwm4 is running but I don't see anything. I can't see any errors in any
of the logs (kernlog, syslog) except for the following warning at the
end of ~/.xsession-errors

(xfwm4:1734): xfwm4-WARNING **: Top strut value for application window
0x1c00004 confined to 0

I'm using a H55 intel board with HDMI output. When I start mythbuntu in
recovery mode, I get a full X session and the warning isn't present in
~/.xsession-errors

How can I find out what is going wrong?

Thanks,
John.