Comment 1 for bug 1025168

This is not a problem for any of the other Horizon devs (almost all of whom are using ubuntu 12.04 or 11.10), so that means there's something going on with your installation in particular.

Googling for "python exec format error" turns up a variety of things you might examine to see if they apply to your case.

Report back if you find out any more information about what's going on.