It's impossible to authenticate in Gnome 3's lock screen, at least on Fedora. Reported upstream:
From the crashes, it looks like it is blindly assuming it was started from GDM and has delegated responsibility for authentication to it.
Upstream claims to have fixed this now:
Will have to retest once that version of gdm has trickled down to the distributions.
This fixed version of gdm now active in RHEL 7 etc.
The version of Gnome that includes this fix is 3.14.