Last night I upgraded Xubuntu 18.04 with the latest package, but there are some error notifications when new packages are installed. Based on info from system notifications, the Window Manager (xfwm4) that I use on Xubuntu crashes. And this causes the desktop environment (xfce4) on Xubuntu become an error.

Source: LXer – How to fix a broken Window Manager(xfwm4) on Xubuntu?