After opening the laptop lid and signing in, the screen blanks and locks once after a delay, even though the computer is in use. Most of the details can be found https://forum.xfce.org/viewtopic.php?id=13488 or even https://bugs.launchpad.net/ubuntu/+source/xfce4-screensaver/+bug/1852817.
*** This bug has been marked as a duplicate of bug 16327 ***