After updating my multiseat-setup was broken, so that lightdm would refuse to start. I could start xfce with startx, but only one seat worked.
Today i solved the problem by disabling 2 config-files which were brought by recent updates to /usr/share/X11/xorg.conf.d These configfiles may break all multiseat-setups using loginctl see arch-multiseat
in short all you have to do is disabling these files:
sudo mv /usr/share/X11/xorg.conf.d/10-amdgpu-nvidia-drm-outputclass.conf /usr/share/X11/xorg.conf.d/10-amdgpu-nvidia-drm-outputclass.conx
sudo mv /usr/share/X11/xorg.conf.d/10-intel-nvidia-drm-outputclass.conf /usr/share/X11/xorg.conf.d/10-intel-nvidia-drm-outputclass.conx
if you are interested in details see the full info at Multiseat-Setup broken by …