I think space is treated special in some ways (it's supposed to be used only as binded in standard config), since i think some other binding also didn't work, but can't remember what that issue was (and it would be another bugreport).
At least i was able to bind "Move back" to Space when unbinding all other actions from it.
EDIT: Just found out that when binding "Right Ctrl" to "Adjust" adjusting down (right crtl+space) works. So I think it doesn't work because of the "hard coded" default binding of the combination "Left Crtl"+"Space". But why is there a stadard binding that can not be overridden?