PS Maybe this problem is related to bug #757081, because I realize I do not have a display-manager.service file:
# ls -l /etc/systemd/system/display-manager.service ls: cannot access /etc/systemd/system/display-manager.service: No such file or directory -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
