Thanks for replying!
So my pic in photoshop is 5760×3840. I export it to AE as a .vpe file.
In AE i import the .vpe file and double click the ready made composition which has the same size (and corresponding frame size).
Pixel aspect is set to 3:2 (1,5) (its not locked)
I never looked into this pixel ratio and learned it so maybe thats the problem?
Looking forward hearing back from you!