Development Journal of a Raytracer

Rendered Image

I rendered the image and there are a lot of problems. First for reference the small image to the left is a link to the full image. Below I will try to show what I noticed.

There are five problems in the image I have marked them in red.

1. I don't understand the stripe on the back wall. The wall is a simple plane with only one material, between light source and wall there is only the box. But that stripe seems to be right in the middle of the plane, there it would cross the z-axis in standard alignment.
2. Similar to 1 and probably related. The hard boundary seems to be at the same place where the line in 1 is.
3. I have no clue what happened here. The Triangle is completely strange, the wall is reduced to two triangles for rendering, but they should have nothing to do with that triangular shape.
4. First BRDF problem. I don't like that hard boundary. It doesn't seem to be a triangle boundary either, but there is something similar in the top corner I just noticed.
5. I have to check if the white highlight is correct. The light is behind the camera, so it could be correct, I just want to be sure.