[LightDM] [1.6.0 and 1.7.2] user session stops immediately

[tjoen]

Lightdm-1.7.2 (same problem with 1.6.0)
# lightdm
after user login lightdm-gtk-greeter immediatly reappears

System linux-x86 LFS-like
No problems with
$ startx
~/.xsession contains
exec startxfce4

$ journalctl
...
Jun 17 11:09:01 localhost login[8606]: pam_unix(login:session): session 
opened \
     for user root by LOGIN(uid=0)
Jun 17 11:09:01 localhost login[8607]: ROOT LOGIN  on '/dev/tty2'
Jun 17 11:09:05 localhost lightdm[8621]: 
pam_unix(lightdm-greeter:session): \
     session opened for user lightdm by (uid=0)
Jun 17 11:09:05 localhost systemd-logind[125]: New session c1 of user 
lightdm.
Jun 17 11:09:05 localhost systemd-logind[125]: Linked /tmp/.X11-unix/X0 to \
     /run/user/92/X11-display.
Jun 17 11:09:12 localhost lightdm[8621]: 
pam_unix(lightdm-greeter:session): \
     session closed for user lightdm
Jun 17 11:09:12 localhost lightdm[8641]: pam_unix(lightdm:session): 
session \
     opened for user tjoen by (uid=0)
Jun 17 11:09:12 localhost systemd-logind[125]: New session c2 of user tjoen.
Jun 17 11:09:12 localhost systemd-logind[125]: Linked /tmp/.X11-unix/X0 to \
     /run/user/1000/X11-display.
Jun 17 11:09:12 localhost lightdm[8641]: pam_unix(lightdm:session): 
session \
     closed for user tjoen
Jun 17 11:09:12 localhost systemd-logind[125]: Removed session c2.
Jun 17 11:09:12 localhost kernel: mtrr: no MTRR for b0000000,10000000 found
Jun 17 11:09:13 localhost lightdm[8654]: 
pam_unix(lightdm-greeter:session): \
     session opened for user lightdm by (uid=0)
Jun 17 11:09:13 localhost systemd-logind[125]: New session c3 of user 
lightdm.
...

$ ls -ld /var/lib/lightdm
drwxr-xr-x 4 lightdm root 4096 Jun 17 11:02 /var/lib/lightdm
default /etc/pam.d/lightdm*

lightdm-gtk-greeter-1.5.1
$ ls -l /usr/share/xgreeters/default.desktop
lrwxrwxrwx 1 root root 27 May 20 12:20 
/usr/share/xgreeters/default.desktop \
     -> lightdm-gtk-greeter.desktop
default /etc/lightdm/lightdm-gtk-greeter.conf

accountsservice-0.6.30
systemd-200

What am I doing wrong?