Hello.
The daytime render looks fine.
As for the night time render, the complete render looks very different from the preview (the scene is far brighter)
I have no clue about what's going on and i have never seen such a problem...
Thanks in advance for any answers.
Here's a screenshot:
It looks like maybe one of your lights (sun light) is hidden in the viewport but not in the render. Check the outliner to see if the eyeball icon is greyed-out on one of your lights while the camera icon is not greyed out. That would make an object hidden/inactive in the viewport but visible/active in the render.