I've just upgraded from 14.04 to 16.04.1 after the prompt from the update manager.
Most of the upgrade seems to have gone alright (Connectivity etc. is just as before). However, my desktop background is black, and it is not changing if I modify it via system settings. It is not possible to right-click on it. Once the desktop background was actually stuck on an image of the login screen (that took me a while to figure out, I thought I hadn't logged in!). A restart brought back the black desktop.
So I'm suspecting something went wrong with Unity. But what? Where could I check this?
Edit: The problem seems to be related to the external monitor. If I boot without it, the desktop background is there. Once the laptop is attached to the external monitor the background becomes black.