Files on my desktop are hidden automatically. What can i do?

Hello Community!

I really don't know what to do. Since a few weeks all my desktop files are hidden automatically. If new files will be added to the Desktop they won't show up after a reboot. All files still appear in the /Users/xx/Desktop/ directory and they show up when I am using the key combination SHIFT+CMD+PERIOD.

Do you have any idea to deactivate this? I really have no clue why the desktop files are hidden and couldn't find anything on the internet about it.

My OS version: macOS Sierra 10.12.5

Thank you guys for any help & time :-)