It helped!
Shall it remain on the desktop?

I still have the problem. I have had the computer(mac)-image put up on a screen, but it has nothing to do with that - in that case, both screens show the same.
What to do then?

Yes, exactly

I am working from a Mac and having trouble with an inactive course-button. A little over a year ago, it worked perfectly - and then suddenly stopped. Nothing happens, when I press the button. Then I was told, that upgrading to the Yosemite system for Mac should solve the problem. It hasn't. What to ...