I think this is the right place for my problem, since I read something similar to this here. Maybe a bit more confusing.
After a bit more than eight hours of trying to figure out it by myself I'm really in desperation.
I tried to learn about camera tracking with the help of a YT tutorial from Blender Guru (https://www.youtube.com/watch?v=CVPcT0dJmoY). Everything went fine till this one point when I again tried to render the scene to check the results. The problem: my background clip disappeared. So far - nothing special. I was sure that I did something wrong and just tried to figure out what it was.
Thankfully I safed a working copy before I started to refine my "hole in the street". A copy, where I just put a cubic hole in the street instead of a more complex shape.
This copy is still working. But the more advanced version doesn't. I spent a whole day of searching and comparing the two versions. And I'm pretty sure I am overlooking something. But I still can not find the mistake I certainly made.
One weird thing I need to mention: after the rendering the background sometimes "blinks" and shows up for a millisecond. But thats all.
I attached the picture of the failed version, since I am not allowed to add two pics in my post. But there is a successfull rendered simple "cubic hole" version.
I checked all the nodes, the scene settings and what not. I really hope someone in this great community is able to help!