I'm using a laptop with a docking station. The following problem occurs when the laptop is docked AND xfce4-power-manager is installed. I can't properly shutdown, reboot or logout the PC, whatever way I choose to do it: panel button, power button, "sudo reboot" in console. Instead, the PC either goes back to login prompt or enters standby mode. It appears that I have "no right" to reboot/shutdown/logout properly. A more detailed discussion has taken place there: https://forum.xfce.org/viewtopic.php?id=9653 I observed this with xfce4-power-manager versions 1.4.1 and 1.4.4.
I have observed a similar problem with Xubuntu and Manjaro Linux on a Thinkpad T520 with Nvidia/Intel Optimus. Undocked & Intel & Notebook display only: no problems, e.g. shutdown, restart etc. worked. Undocked & Nvidia & Notebook display only: no problem. Undocked & Nouveau & Notebook display only: no problems. (Docked & Intel & External monitor: could not test, because I didn't get Optimus to run, which is needed because DVI is hard-wired to nvidia.) Docked & Nouveau & External monitor only: no problems. Docked & Nvidia & External monitor only: shutdown, reboot brings me to the login screen. 'shutdown now' works, but the next poweron hangs somehow. Have to turn off & on again, then it works. In my case I could narrow it down to the proprietary nvidia driver. So for me only the Nouveau driver works in every case.
Sorry this report didnt get anyone's attention for so long but it also seems a little "isolated" if I may say so. There are lots of people with suspend issues etc and most of them are related to graphics drivers, not xfce4-power-manager. In any case, it's been a long time, I'm closing this report now (ToZ already tried to get to the bottom of it, I don't think I'll be able to do more without having *your exact hardware setup*).