TigerVNC fails to unlock session

vncviewer connects to the server running lightdm and offers two options:

  1. login to a new session works readily
  2. unlocking a running session fails with the display manager on the server reporting:
 6700k:~ # journalctl -b -u display-manager.service --since 8:32
Sep 09 08:34:17 6700k display-manager[16484]: /usr/bin/xrdb: Can't open display ''
Sep 09 08:34:17 6700k display-manager[16485]: /usr/bin/xset:  unable to open display ""
Sep 09 08:34:17 6700k display-manager[16486]: /usr/bin/xmodmap:  unable to open display ''
6700k:~ # 

Any idea?