CentOS - Aug 2018 - Centos 7.5..and My GDM Greeter/GDM Don't Work Anymore

So, we've updated to 7.5, and for two machines--A Dell Precision M4800, 
and a Dell Precision 7510..both running Radeon cards--we no longer get 
the GDM Greeter login box NOR can we switch to virtual text login 
terminals. It's been driving me nuts. The systems do allow for SSH 
logins, so that's how we get to them, but I've checked the Xorg.0.log 
and nothing stands out. Did a systemctl status gdm.service and got this:
> Aug 07 16:23:31 gs6102dsclxpvirgo3 gnome-shell[2023]: g_array_unref: 
> assertion 'array' failed
> Aug 07 16:23:31 gs6102dsclxpvirgo3 gnome-shell[2023]: g_array_unref: 
> assertion 'array' failed
> Aug 07 16:23:31 gs6102dsclxpvirgo3 gnome-shell[2023]: g_array_unref: 
> assertion 'array' failed
> Aug 07 16:23:31 gs6102dsclxpvirgo3 gnome-shell[2023]: g_array_unref: 
> assertion 'array' failed
> Aug 07 16:23:31 gs6102dsclxpvirgo3 gnome-shell[2023]: g_array_unref: 
> assertion 'array' failed
> Aug 07 16:23:31 gs6102dsclxpvirgo3 gnome-session-binary[1926]: 
> WARNING: Could not get session path for session. Check that logind is 
> properly installed and pam_systemd is getting used at login.
> Aug 07 16:23:31 gs6102dsclxpvirgo3 gnome-shell[2023]: g_array_unref: 
> assertion 'array' failed
> Aug 07 16:23:31 gs6102dsclxpvirgo3 gnome-session[1926]: 
> gnome-session-binary[1926]: WARNING: Could not get session path for 
> session. Check that logind is properly installed and pam_systemd 
> is...ed at login.
> Aug 07 16:23:31 gs6102dsclxpvirgo3 gnome-shell[2023]: g_array_unref: 
> assertion 'array' failed
> Aug 07 16:23:32 gs6102dsclxpvirgo3 gnome-shell[2023]: JS ERROR: 
> Exception in callback for signal: reset: Gio.DBusError: 
> GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: No session available
> LoginDialog<._resetGreeterProxy at
resource:///org/gnome/shell/gdm/loginDialog.js:837:29
> wrapper at resource:///org/gnome/gjs/modules/_legacy.js:82:22...
Did the upgrade break something? Possibly. Any and all hints/ideas/etc. 
greatly welcomed.

PEV

-- 
Paul E Virgo
Sr. System Administrator
Code 610.2
SESDA III - DAAC/DISC
Goddard Space Flight Ctr/NASA
Greenbelt, MD
Greenbelt, MD 20771
(301) 614-5751