linux/xfce4: screensaver stopped working, seeming idle detection issue

I'm using xfce4 under ubuntu with the 4.15.0.30 kernel.

Starting yesterday after a number of reboots, the screensaver (I'm using xscreensaver) stopped getting invoked when the system is idle.

• xscreensaver is definitely running
• When I issue xscreensaver-command activate the screensaver starts successfully
• When I close the laptop lid and reopen, xscreensaver comes up properly with a lock screen.
• The screensaver is enabled within the xfce4 settings panel and set to blank after 3 minutes. No matter what value I use for "Blank after", the screen saver never automatically pops up.
• I can issue a Preview of the screensaver within the settings panel, and it works.
• I have completely uninstalled and reinstalled xscreensaver and completely reset all of its settings. This did not fix the problem.
• I have rebooted several times, and this did not fix the problem, either.

My guess is that this isn't a problem with xscreensaver or its configuration, but rather, it's as if my system is no longer detecting that it is in an idle state. How do I test and diagnose this? What factors can cause my system to stop realizing that it is idle?

I made a change in /etc/systemd/system.conf yesterday. I entered this setting:

DefaultTimeoutStopSec=15s

... and then I issued systemctl daemon-reload and rebooted. All other entries in /etc/systemd/system.conf are commented out. Thinking that this might be causing the problem, I commented out that setting again, ran systemctl daemon-reload again, and then followed that by a reboot. But the same screensaver problems continued.

Any ideas of what might be causing my system to not detect that it is idle, and how this might be fixed?

Thank you in advance.