Hello Chris von Simson.
Thank you for using Apple Support Communities. It looks like you're experiencing issues with your iCloud Drive not allowing you to properly preview items and not move items to your desktop. If this is the case, we'd like to look into this issue with you.
Ever since this issue started to occur, have you been able to connect to a different reliable Wi-Fi network to see if this specific issue continues to occur? Does restarting your Mac mini help?
Log out, sleep, wake, restart or shut down your Mac
If restarting doesn't help, there are some additional troubleshooting steps you can attempt. We can try starting up your Mac in safe mode. Safe mode is a great step to try to help further isolate issues that might arise when using your Mac. Check out the following resource:
How to use safe mode on your Mac
Just follow the steps listed under "How to use safe mode". You can also create a new user on your Mac and test to see if this issue is user specific. For steps on how to create a new user on your Mac, click here:
Set up users, guests, and groups on Mac
We hope this information is useful. Best of luck.