My first two guesses. 1: The images are not precisely lined up on the pixel grid causing the edges to antialias. 2: The images do not completely fill the frame - there are black pixels on the edge.
Check by setting magnification (far left option in the composition panel) to 800% or higher and view the edges. Make sure view transparency grid is turned on.
If there are black pixels crop or mask your images to remove the edges. If the edges do not line up precisely then overlap the images slightly or carefully align them.
You're also working with your transparency disabled which is not a good idea. The little checker board just the left of "active camera": click it to enable transparency.
Thank you a lot for the reply,
So I guess you were right on some parts but,
the black line is not part of the original footage/scene so maybe I have some wrong setting with the transition (what has to do with the keyframes etc),
Or like you said that it's not lined up precisely, if so how could I fix that?
Oh thank you for the tip
Just a question, what is the purpose of it?
Well, when it's not enabled (which it isn't by default) After Effects creates a "false" background or matte.
It's fake because it does not render. It only exists in the preview.
So what happens is new users will create a new project and think they're giving it a blue background when they're not.
I suppose it has its occasional use while working but I can't see any logical benefit in having disabled by default.
Mostly, the only thing it does is confuse new users.
thanks alot for the tip then since I'm pretty new and still learning .
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Yes I did, thank you for your concern.
It was the clip that was the problem, pretty dumb.