If you're running XFCE then the solution is pretty simple, deactivate xfce-power-manager's presentation mode. In presentation mode the xfce-power-manager is sending keystrokes to keep the display awake. You can disable this mode by right clicking on the tray icon or by running:
$ xfconf-query --channel xfce4-power-manager --property /xfce4-power-manager/presentation-mode --set false