Unity – Recursive textures do not work in 2018.3

I have a project in which I am playing with a video feedback effect when I render a camera to a RenderTexture and point the camera to a quad that shows that RT. This usually works fine, but when you try HDRP, LWRP (and update the materials) or update the entire project to 2018.3, the RT simply stops working and displays in black. I also tried a completely new project and only had a camera, a quad and a cube. That did not work either.

Is there any obvious reason why this does not work? I spent some time trying several configurations but I could not find anything that had any effect, so I really do not know where to start.