Strange Canvas behaviour at Screen Space - Camera mode?

Hey folks,

when I updated one of my older projects to Unity 5.5, I noticed, that the whole UI was pretty messed up. The scaling was completely different - nothing matched the screen. The solution to that was changing the canvas mode to Screen Space - Overlay. For my new project I wanted to add image effects to the UI by setting the mode to Screen Space - Camera. Then I had to realize, that in this "clean" project, the Canvas was messed up, too. What am I doing wrong?

Thanks for your help!