Hi Tobias,
Thank you for querying in this forum.
From your description, it seems that you encountered an window “what’s new in Excel” after fresh installation on Mac. And when you click Okay, nothing happens, but if you closed the window, Excel will be closed.
If my understanding is right, generally, if we activate Office, it will appear What’s new in Excel, then we can click Continue and it will appear Your privacy option, after clicking Ok, we can open Office application without any problem.
And generally, this dialog will appear when you use Office application for the first time or update Office. If you just close the window, it will close your Office applications directly. You may need to click Ok.
In your scenario, you mentioned “I can click on the OK-button, but nothing happens”, does it mean that it will not appear the window to open Office or you still see the dialog? Could you provide more details about your situation?
At the same time, could you provide the version of your office? You can go to Finder > Applications > Excel and check the version.
Best Regards,
Sukie