lightdm black screen after installing plasma5 in Ubuntu

I have both ubuntu-desktop and kubuntu-destop installed on my laptop. While sddm works perfectly fine during login, if I choose lightdm as the default login screen, after a reboot, I get a black screen. The black screen goes away if I remove Kubuntu completely and only gnome files are present.

How do I fix this black screen issue?

OS: Ubuntu 15.04 x64 with Unity 7 and Plasma 5

had the same issue, just removing sddm was enough to get lightdm working again

apt-get remove --purge sddm

searching for the problem brought up some issues regarding the coexistence of lightdm and sddm -> since you'll almost always only need one login-manager anyway, removing sddm should be OK to solve this problem