Issue with "Black Boxes" appearing

[Germenfer]

Hello,
I am having an issue regarding "black boxes" appearing out of nowhere in my
latex file whenever a new page inside a chapter is started due to the text
being too long for the previous page. This can be seen in IMAGE 1 (before
adding new content, there is no black box) and IMAGE 2 (after new content
is added and a new page is created, a black box appears). Proof of this
being a strange occurrence can be seen in IMAGE 3, where the page-skip does
not add a black box.
Also, it appears that this "black box" is a standalone entity, as it can be
deleted without affecting anything else on the document using Adobe Reader.

I would greatly appreciate some help because it has been driving me crazy
for a while now.

Kind regards,
Juan.
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