Shutdown freeze stop power off on Lxde Lubuntu12.04

Days ago when I wrote "Ubuntu Lubuntu 12.04 Shutdown doesn't power off" post, I changed the "quiet splash acpi=force" to "quiet splash" and saved the text file. Today I have face same problem again. When I trying to shut down computer it goes freeze or hang at a stage and displayed the message:

modem-manager[4971]: Caught signal 15, shutting down...
modem-manager[4971]: modem /org/freedesktop/ModemManager/Modems/0: state changed (registered -> disabling)
modem-manager[4971]: (ttyACM0) closing serial port...
modem-manager[4971]: (ttyACM0) serial port closed
modem-manager[4971]: modem /org/freedesktop/ModemManager/Modems/0: state changed (disabling -> disabled)
[ 6457.7700631] System halted.
_

Here is the photo (taken by mobile phone) of the message:

Shutdown freeze stop power off on Lxde Lubuntu12.04

I follow the same instruction and change "quiet splash"  to "quiet splash acpi=force" and saved the file. It solved my problem. My computer goes power off normally.