I installed lightdm along with the gtk3 greeter.
Upon trying to start cinnamon it failed, however it did work in Software Rendering mode.
Gedit still opened from the menu.
Running cinnamon-settings seems to have worked.
python-pam is installed for me as well, the 2.x version NOT the 3.4 version.
pam, pam-base, pam-libs as well.
Perhaps try to reinstall the pam packages? I'm sorry, but I really have no idea what else it could be and I can't seem to replicate your issue on my machine. My machine is also far from fresh.
If you can replicate the issue on a fresh install, I'd be happy to try it myself. I should be getting a Lenovo T420 in a couple days, I'll probably want to install void anyway. If I remember I'll try to test it out then as well.