I’ve got some memory-intensive works to do recently. So I managed to log in to lxde desktop again last night after repairing a long-pending black screen problem by just uninstalling Nvidia driver.

However, I came up with a new problem this morning. After I set wallpaper on an external monitor, the desktop-preference program crashed by reporting "desktop manager is not active" and I got a wrongly configured wallpaper as an accompanying result. The link here said that this is because the pcmanfm failed to draw desktop at startup. Manually redraw desktop by executing:
pcmanfm --desktop &
will restore desktop-preference. I planned to add this command to autostart menu and found that there was already a similar command:
pcmanfm --desktop --profile LXDE
Obviously, this is the failed command which caused the trouble. It seems that an LXDE profile config file related to this command was corrupted.
I finally found this desktop-items-0.conf file under /home/user/.config/pcmanfm/LXDE and modified the wallpaper related settings in it to default. To be specific:

wallpaper_mode=crop
wallpaper=/usr/share/backgrounds/images/default.png

Actually, I copied these 2 lines from a desktop-items-1.conf file in the same folder.