I had the problem that when shutting down, Debian Linux (stretch) only halts („System halted“), but does not switch off the computer. Strangely enough, it works if you shut down from the console:
sudo poweroff
The cause was that kdm, which does no longer exist on Debian stretch, was still installed and used, instead of the new sddm.
Running
sudo apt-get install sddm kdm-
to remove kdm and install sddm fixed the problem.