Have you got the package budgie-desktop-view installed? It shouldnt be if you have nemo managing your desktop.
Hello, I think so. I’m still checking. I think I just found the error! I want to document this in peace. Let me know tomorrow…
So, I have the desktop running again!
What I found. I spent days searching and correcting in the dconf-editor. Now I came across the settings under /org/budddiesofbudgie/budgie-desktop-view. There I wondered why the parameters show, show-home-folder and show-trash-folder were active. I deactivated them and everything works again! 
This certainly coincides with @fossfreedom’s indication that the budgie-desktop-view package is installed. After uninstalling it, the dconf settings had also disappeared. Now it works for all users.
Thank you ALL for helping me!!! Once again I have learned a lot. 
Greetings!!