nodm unsucessfull in terminating X server on failure

We use 'nodm' in our diskless kiosk-system, where multiple clients boots from the same NFS-share.

Running Ubuntu 16.04.2 LTS
Installed nodm from Ubuntu repo, version 0.11-1.3ubuntu1

I have modified the nodm.service to get better control of the process (The faulting was also happening with original nodm.service, but it was less visible):

[Unit]
Documentation=man:systemd-sysv-generator(8)
SourcePath=/etc/init.d/nodm
Description=LSB: No Display Manager
Before=multi-user.target
Before=multi-user.target
Before=multi-user.target
Before=graphical.target
After=console-screen.service
After=kbd.service
After=hal.service
After=remote-fs.target
Wants=cups.service
RequiresMountsFor=/home/bruker /var/lib/xkb/

[Service]
EnvironmentFile=-/etc/default/nodm
ExecStart=/usr/sbin/nodm $NODM_OPTIONS
Restart=always
TimeoutStopSec=5s
KillMode=mixed

This works on 20 serveres, with 5-50 clients connected.

But at some random time, X is unable to start on a client.
NODM tries to terminate Xorg, but doesn't do so sucessfully.

root@publikumpc:/home/bruker# systemctl status nodm.service
● nodm.service - LSB: No Display Manager
   Loaded: loaded (/etc/init.d/nodm; enabled; vendor preset: enabled)
   Active: active (running) since on. 2017-06-07 15:11:54 CEST; 15min ago
     Docs: man:systemd-sysv-generator(8)
 Main PID: 855 (nodm)
   CGroup: /system.slice/nodm.service
           ├─855 /usr/sbin/nodm
           └─858 /usr/lib/xorg/Xorg :0 -nolisten tcp vt7

juni 07 15:11:54 publikumpc nodm[855]: Current version of pixman: 0.33.6
juni 07 15:11:54 publikumpc nodm[855]: Before reporting problems, check http://wiki.x.org
juni 07 15:11:54 publikumpc nodm[855]: to make sure that you have the latest version.
juni 07 15:11:54 publikumpc nodm[855]: Markers: (--) probed, (**) from config file, (==) default settin
juni 07 15:11:54 publikumpc nodm[855]: (++) from command line, (!!) notice, (II) informational,
juni 07 15:11:54 publikumpc nodm[855]: (WW) warning, (EE) error, (NI) not implemented, (??) unk
juni 07 15:11:54 publikumpc nodm[855]: (==) Log file: "/var/log/Xorg.0.log", Time: Wed Jun 7 15:11:54
juni 07 15:11:54 publikumpc nodm[855]: (==) Using system config directory "/usr/share/X11/xorg.conf.d"
juni 07 15:12:54 publikumpc nodm[855]: X server did not respond after 60 seconds
juni 07 15:12:54 publikumpc nodm[855]: sending X server 858 the TERM signal

nodm is "sending X server 858 the TERM signal"

That doesn't solve anything.

If I run "kill -9 858" as root, everything is OK again:

● nodm.service - LSB: No Display Manager
   Loaded: loaded (/etc/init.d/nodm; enabled; vendor preset: enabled)
   Active: active (running) since on. 2017-06-07 15:27:28 CEST; 7s ago
     Docs: man:systemd-sysv-generator(8)
 Main PID: 1139 (nodm)
   CGroup: /system.slice/nodm.service
           ├─1139 /usr/sbin/nodm
           └─1145 /usr/lib/xorg/Xorg :0 -nolisten tcp vt7

juni 07 15:27:28 publikumpc nodm[1139]: (++) from command line, (!!) notice, (II) informational
juni 07 15:27:28 publikumpc nodm[1139]: (WW) warning, (EE) error, (NI) not implemented, (??) un
juni 07 15:27:28 publikumpc nodm[1139]: (==) Log file: "/var/log/Xorg.0.log", Time: Wed Jun 7 15:27:28
juni 07 15:27:28 publikumpc nodm[1139]: (==) Using system config directory "/usr/share/X11/xorg.conf.d"
juni 07 15:27:32 publikumpc nodm[1139]: The XKEYBOARD keymap compiler (xkbcomp) reports:
juni 07 15:27:32 publikumpc nodm[1139]: > Warning: Type "ONE_LEVEL" has 1 levels, but <RALT> h
juni 07 15:27:32 publikumpc nodm[1139]: > Ignoring extra symbols
juni 07 15:27:32 publikumpc nodm[1139]: Errors from xkbcomp are not fatal to the X server
juni 07 15:27:32 publikumpc nodm[1159]: Successful su on ??? for bruker by root
juni 07 15:27:32 publikumpc nodm[1159]: pam_unix(nodm:session): session opened for user bruker by (uid=