phantom DM in 15.10

[Felix Miata]

I have a 15.10 installation on intel 945G in which ps -A reports sddm
running, but there is no GUI screen to be found on any vtty. Startx works,
creating a session on :1 (on whichever vtty logged into), since the phantom
DM is laying claim to :0 on vtty7. sddm.log shows sddm-helper exited and
greeter stopped, which is a poor clue to where the problem lies. Systemctl
confirms sddm running:

● sddm.service - Simple Desktop Display Manager
   Loaded: loaded (/lib/systemd/system/sddm.service; enabled; vendor preset:
enabled)
   Active: active (running) since Sun 2016-02-21 22:50:52 EST; 10min ago
     Docs: man:sddm(1)
           man:sddm.conf(5)
 Main PID: 766 (sddm)
   CGroup: /system.slice/sddm.service
           ├─766 /usr/bin/sddm
           └─808 /usr/bin/X :0 -auth /var/run/sddm/:0 -nolisten tcp
-background none -noreset vt7

Feb 21 22:50:52 gx620 systemd[1]: Started Simple Desktop Display Manager.
Feb 21 22:50:54 gx620 sddm-helper[810]: pam_unix(sddm-greeter:session):
session opened for user sddm by (uid=0)

Googling seems to indicate this problem has to do with PAM, but nothing I've
found so far has described a solution that works here. /var/log/sddm.log also
points to PAM:

(II) DAEMON: Initializing...
(II) DAEMON: Starting...
(II) DAEMON: Adding new display 0 on vt 7 ...
(II) DAEMON: Adding cookie to "/var/run/sddm/:0"
(II) DAEMON: Display server starting...
(II) DAEMON: Running: /usr/bin/X :0 -auth /var/run/sddm/:0 -nolisten tcp
-background none -noreset vt7
(WW) DAEMON: Signal received: SIGUSR1
(II) DAEMON: Running display setup script  "/usr/share/sddm/scripts/Xsetup"
(II) DAEMON: Display server started.
(II) DAEMON: Socket server starting...
(II) DAEMON: Socket server started.
(II) DAEMON: Greeter starting...
(II) HELPER: [PAM] Starting...
(II) HELPER: [PAM] Authenticating...
(II) HELPER: [PAM] returning.
(II) DAEMON: Greeter session started successfully
(II) HELPER: [PAM] Ended.
(WW) DAEMON: Auth: sddm-helper exited with 2
(II) DAEMON: Greeter stopped.

Any ideas how to fix this?
-- 
"The wise are known for their understanding, and pleasant
words are persuasive." Proverbs 16:21 (New Living Translation)

 Team OS/2 ** Reg. Linux User #211409 ** a11y rocks!

Felix Miata  ***  http://fm.no-ip.com/