Desktop is broken
At first please don't blame me for my English. I'm German so I have to fix my problem with the German keyboard layout.
What I tried to do: I was trying to change Wayland to x11 because of Wallpaper Engine (Steam). I have changed it and tried in the system settings to set up the right location of Steam (for Wallpaper Engine). It was a bit tricky for me but after many tries it worked (needed to make hidden folders visible).
I still don't know why I couldn't see all my Wallpaper Engine Wallpapers (first problem but not the reason why I'm writing here).
I tried a few Wallpapers and...my Desktop crashed ... Black Screen
I only can see my mouse 
cursor.
What I tried to do: I was trying to change Wayland to x11 because of Wallpaper Engine (Steam). I have changed it and tried in the system settings to set up the right location of Steam (for Wallpaper Engine). It was a bit tricky for me but after many tries it worked (needed to make hidden folders visible).
I still don't know why I couldn't see all my Wallpaper Engine Wallpapers (first problem but not the reason why I'm writing here).
I tried a few Wallpapers and...my Desktop crashed ... Black Screen
I only can see my mouse cursor.