Reset the taskbar of KDE to default settings, this is how it works.
Quickly it happened, something pushed back and forth, then a part was gone. Later, I still had an analog clock in the taskbar.
Fortunately, the whole thing can be quickly reset to the default settings. To do this, we open a console and run the following three commands.
rm ~/.config/plasma-org.kde.plasma.desktop-appletsrc rm ~/.config/plasmashellrc rm ~/.config/plasmarc
The last one may cause an error, just ignore it. Afterwards, log out and log in again. The taskbar is now back with default settings at the bottom.