So there are two problems here. Compiz no longer ships a file called compiz.real and gnome-session doesn't have any kind of fallback when the WM saved in your session no longer exists. A hack in the compiz package to ship a symlink would fix this one instance of the problem but a fix in gnome-session would be the correct solution.
So there are two problems here. Compiz no longer ships a file called compiz.real and gnome-session doesn't have any kind of fallback when the WM saved in your session no longer exists. A hack in the compiz package to ship a symlink would fix this one instance of the problem but a fix in gnome-session would be the correct solution.