No screen saver on external monitors

[KatarzynaCzaja]

Did you make sure to enable the screen saver on all spaces?
In Screen Saver setting right next to the screen saver preview on the top you should see a slider that says 'show on all spaces' and if you have that greyed out you will have an option to select on which screen the screen saver will show (right under the screen saver preview). If you enable the slider it should show the screen saver on all connected monitors.