The Dell Optiplex 755 misbehaves for a lot of users,
under Linux, Microsoft Windows (various versions), Solaris, etc.
The problem is, it often does not power off completely at system shutdown.
A quick online search reveals that many people have been perplexed by this issue
over the years. A similar, related issue, is that it also sometimes fails to reboot
when told to do so by the end-user.
How to Fix Poweroff at Shutdown on Dell Optiplex 755
The cause of this problem is the hidden MEBx BIOS in the box,
which is designed to keep the machine alive enough for remote management,
including remote startup and shutdown.
Disabling the MEBx completely will fix the power off and reboot issues
in most cases.
First, restart the computer, and hit F2 to enter the main BIOS setup menu.
Scroll down to Post Behaviour, and from there enable the MEBx Hotkey.
Escape/Save, and the system will restart. This time hit Control^P to get into the MEBx BIOS settings screen.
The default password for this is admin and you will be forced to enter a new, complex password before it will allow anything else to be changed.
Once you have set a new complex password, write it down, preferably on/in the computer itself with a permanent marker.
Now, you can disable "State Control" in the MEBx settings. Do that, and quit out.
Restart the system, hit F2 again, and disable the MEBx Hotkey setting.
Escape/Save, and the system should now behave as intended, including powering off reliably at system shutdown.
When setting the complex MEBx password, the firmware requires that it abide by these rules:
eight characters, including:
one capital/uppercase letter
one small letter
one (non-alphanumerical) additional character (underscores don't count). Eg. ! or $ or ;
must not contain any periods or double-quote characters