What can disable the UI from being used?

I've checked for errors, used Debug.Log etc. and I can't use the UI in the play mode. When I click play and it's loaded the scene it looks like there's a wall in between the mouse and the UI elements. The UI consists of buttons, images, text and panels, but the buttons don't even change when I hover over them with the mouse.

I've tried replacing the Canvas, Eventsystem, Camera and replacing a (verified) working backup with the new scripts, art etc. So I know something is wrong with the scripts or the setup of the UI, but I don't know what could interfere...