The pop-up window appeared after I clicked on the "start share screen" button. However, when I click on the "share" button in the pop-up, nothing happens. What can I do?
You have to select the display you want to share in the pop-up window - even if you only have one display. You click on the display, and the display will get a blue border around it. The "share" button will then also turn blue and you can click it to start the screen share feature.