Well, if one wanted to be pedantic, from a photographic point of view, the sun could be viewed as an issue as it is a white blob, that could be seen as distracting by the reviewer - maybe! It does draw your attention away from the rest of the scene, as the eye focuses on the sun.
And maybe exposure in the foreground could be lighter in the corners.
You should not upscale. that's in the instructions. Pictures don't get better in quality by upscaling. Defects get upscaled too and are a lot mor visible at an upscaled size.
Your refusal reason is "Quality issues". That's enough for us.
You really need to study the submission guidelines. You will yourself spare a lot of refusals by correcting issues beforehand. (If they are indeed correctable in a decent way)