following the update (at Jun 22 2017) several Services refuse to start,
messages like those appeared in journal:

systemd[1852]: apache2.service: Failed at step NAMESPACE spawning
/usr/sbin/start_apache2: No such file or directory
systemd[1868]: ntpd.service: Failed at step NAMESPACE spawning
/usr/sbin/start-ntpd: No such file or directory
systemd[3862]: rtkit-daemon.service: Failed at step NAMESPACE spawning
/usr/lib/rtkit/rtkit-daemon: No such file or directory

systemctl status showed (e.g.)
...
  Process: 1852 ExecStart=/usr/sbin/start_apache2 -DSYSTEMD -DFOREGROUND -k
start (code=exited, status=226/NAMESPACE)
 Main PID: 1852 (code=exited, status=226/NAMESPACE)


XFCE Desktop is broken/unusable:

- black background, no desktop icons, no context menue at right click on
desktop
- Thunderbird opens (from saved sesion) but nothing is selectable
- task bar appears where it should be with seemingly all entries
- 'Start' Button works, but only once, 'Start'-Menue pops up, but nothing is
selectable
- <Ctrl><Alt><Backspace> (double) terminates session => greeter reappears - at
least this works


some googling broght up a hint related to symlinked /tmp to /var/tmp
and 
systemctl status systemd-tmpfiles-setup.service

showed something like 'error ... /var/tmp is no directory'

using the recommended method (from man) 
# ln -s /dev/null /etc/tmpfiles.d/tmp.conf

makes the error message from systemctl status systemd-tmpfiles-setup.service go
away but the overall problem persisted

removing the symlink /tmp/ => /var/tmp and making /var/tmp a separate directory
allows me to use the system again, but as I like to have all *tmp* reside
within RAM disks I am now forced to create multiple of those