Open the System Preferences, choose Desktop & Screen Saver, click on MinimalClock and choose OK.
Now, only if the screen saver does not work, open the System Preferences, choose Security & Privacy and, in the General section, click on Open Anyway. Finally click on the preview of the screen saver and then click on Open.
How to uninstall
Open the System Preferences, choose Desktop & Screen Saver, right-click on MinimalClock and click on Delete.