But now, when I try poweroff, there are a lot of output messages and the poweroff process stops. I can power off the PC by holding the power button. I can reboot the PC and everything seems normal. I just cannot power it off completely, either with the Gnome poweroff applet or by "systemctl poweroff".
My PC is an HP Probook 450 G3 and my OS is Fedora 26 64-bit. There is nothing Windows-oriented on the system.