Comment 3 for bug 1583624

What appears to be happening is the greeter is not aware it is running inside Unity System Compositor and so it trying to directly access the video drivers. This fails because it is not running as root.

The cause seems to be LightDM is setting MIR_SOCKET to the name of the U-S-C socket but this no longer works (possibly due to a change in bug 1290345).

The solution is to set MIR_SERVER_HOST_SOCKET instead, which is the recommended/correct variable to tell a Mir server the socket of the parent server (U-S-C in this case) to connect to.