So we may open up a club 
I am also affected.(nouveau, lightdm, xfce) But it does hit not every time.
- One time i updated → no GUI after boot (3 times)
- Rollback → lightdm works 98% (dual seat)
- I start from manjaro-usb without update → OK (but single seat)
- I start from manjaro-usb with update → no GUI (even with single seat)
**So the problem must be in the update ??? **
NO !
- I start from manjaro-usb with update → OK
- Now, when i start from USB, lightdm works 95%. I will have to try the update again.
Update:
SOLVED 
see at Loginctl with second seat leads to crash of xorg/lightdm - #13 by andreas85