Seems to be working just fine for me on v4
Though not completely setup, I can get the camera to launch after pressing power twice (in my screenshot below).
In the background is the section of Xtended settings, under Gestures, where you set this up.
Maybe try switch that option off, restart System UI, then switch it back on... Hopefully that gets it up and working again?
Thanks for your reply, I've tried that but it didn't work.
So I solved that problem by clean flashing ;-)