Comment # 3 on bug 1176338 from Stefan Dirsch

It works fine with xdm itself. I've tested xdm, sddm, ligthdm as
displaymanagers so far. The issue only happened with lightdm.
Reassigning to maintainer of lightdm. @Thorsten In case you have any hints,
what could be the issue, please let us know. Thanks!

Reproduction is rather easy. Just use Tumbleweed, which includes pam package
with /usr/etc/pam.d support. Then move /etc/pam.d/xdm* files to /usr/etc/pam.d
and restart lightdm. When trying to login lighdm shows you 

  "authentificataion failure"