It drove me mad that i logged always out from XGL after pressing accidentally shift-backspace.
You have to type:
xmodmap -e "keycode 22 = BackSpace BackSpace"
after your Window Manager started (that would be beryl in my case).
And because my AltGr key didn't worked too i first looked at the keycode with xev which was 113.
Then a little lookup how the key is occupied:
And finally to make the key work again:
xmodmap -e 'keycode 113 = Mode_switch'
And because i am a lazy person and dont want to type that over and over again to load it on startup i put both in my ~/.Xmodmap file:
echo 'keycode 22 = BackSpace BackSpace' >> ~/.Xmodmap
echo 'keycode 113 = Mode_switch' >> ~/.Xmodmap