LightDM greeter disappears when I plug in my keyboard

[deerangle]

I have a Raspberry Pi 3 running Arch Linux ARM. I installed LightDM, the GTK greeter for LightDM, xorg-server and Xfce. When I start my Pi without a Keyboard plugged in, it starts up and shows me the LightDM greeter asking for a password. When I plug in my Keyboard though, the screen disappears to show the kernel output. Booting with the keyboard plugged in also doesn't work. Plugging in a Mouse has the same effect. I believe this is a software/configuration issue.

Any help with this issue is greatly appreciated.