Hello, I have a plane that is very oblique in space. Now I want to align a camera parallel to this plane (see screenshot). Does anyone have an idea how to do this? 
Maybe you can use the Normal Transform Orientation. So for example, you can place both objects at their origin points so they occupy the same place, and then from there you can easily move the camera to where ever you desire following the normal orientation of your plane. It's hard to explain. I always point to a specific part of a lesson in PotHead where JL deals with something similar. Here's the link, at around 17:50:
https://cgcookie.com/lessons/modeling-the-left-leg-i
You could also use the Copy Attributes Menu addon. Go to Edit and Preferences and in Addons search for Copy Attributes. There you can copy the orientation of the plane to your camera.
Hi Max Max-Erl
A fast alternative is, to select the Camera and then the Plane and then:
It will also move the Camera to the Plane, so you have to correct that. You could, for instance SHIFT+S > Cursor to selected before the alignment. And afterwards, SHIFT+S > Selection to Cursor (both with only the Camera selected of course).
Or just move the Camera by hand.
You'll also probably have to Rotate the Camera 90° one one or more of it's Local Axes.