Different render outcome. cycles

Hello, Fello Blend benders.
So I have ran into a problem that I’m trying to tackle for the third day now. I tried different kind of render properties, but that doesn’t seem to change a lot.

First problem that I have is that when the render is from 1-99% it looks acceptable for a first animation test, but when I go to the folder and open the JPG it looks completely different.

What should I look into to fix this ?
And what is that Black Cube ?

You might need to share the file for someone to really look at what’s happening.

Can you post a screenshot of your outliner and object wireframes?

check your compositior

Here is a link with the blender file - https://we.tl/t-5Pw7DVzTXT

And some extra images of my settings.
P.S. My render time is still really high for the resolution outcome, but I guess this is how it is with smoke sim.

Screenshot 2021-01-22 at 23.34.21Screenshot 2021-02-01 at 13.37.02