Image quality decreases in the rendered area

Hi, I have a clip in my timeline. As soon as I apply an effect and the workspace is NOT pre-rendered, the image is sharp. As soon as I pre-render, the image quality decreases. Even if I actually render the video, it is still blurred. How can it be that it is not pre-rendered sharp and rendered blurred? Makes no sense to me...
This phenomenon does not occur with every video in my timeline. Only with one video and one photo.
I have already deleted the video render files and regenerated them.
I have already re-linked the video.
I have converted the raw material into other formats and re-linked it with Premiere.
It is still displayed pre-rendered blurred and not rendered sharp.

Need help. Thank you!