It's not a usb 2.. 1 problem is it ?
Sometimes the backports are usb 2 and the front ones are usb 1.

You could try a couple of things to check you don't have broken
packages though,
Drop to a console and try sudo apt-get install -f
if there is a broken package this should find it.
Then try sudo dpkg-reconfigure -phigh xserver-xorg

The other thing that may just get it going is
sudo dpkg --configure -a
Failing that you could bite the bullet
and
sudo dpkg-reconfigure -a
Pay careful attention to what you are doing if you choose the last option
though as it basically fully reconfigures your system.