Weird rims of objects

Hello there,
I tried to render a video with UE5 in which I used a landscape with a png image sequence (with alpha parts) as video clip in front of it. (Like: A guy as png sequence with alpha masked out walking in a landscape)
The transition from png image to the landscape around looked seamless in editing mode. But when I rendered the video in 4k without comprimising activated the rim of the png image sequence looked kinda weird.

I used a bokeh with f1.8 in the virtual camera. The bokeh worked but it did not start directly from the rim of the png image sequence. There is a gap between the image and the landscape where the bokeh is not existent but the background is very sharp. So you have: The person in the foreground, than a small part next to it where the background is sharp, and then a few milimeters later the unsharp bokeh background starts. This especially happens then, when the image sequence is slightly out of focus in the virtual clip camera.

How can I get rid of this gap?
+The image sequence looks kinda pixelated and there seems to be no kind of antialiasing for the image sequence. How do get antialiasing for it?