Hi, pingaffe
Did you already figured out? Just in case, I’d like to share how did I do. It’s seems MATE does not have /etc/lightdm/lightdm.conf by default. so I created lightdm.conf and write those line below;
[LightDM]
start-default-seat=false
xserver-allow-tcp=true
[SeatDefaults]
xserver-allow-tcp=false
greeter-session=lightdm-greeter
[XDMCPServer]
enabled=ture
and reboot. Then it should be work. at least my RPI3 with Ubuntu MATE 15.10 is works fine with this configuration.
The point is Xwindow must not be started on RPI3. I mean GUI login. and must allow tcp session.