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I did some additional digging around and found out what was causing the error, which was one of the documents that I was importing also had the commands

\usepackage{}
\begin{document}
\end{documents}

That's what I get for switching from 1 file to multi-file structure, it can be easy to ...

you could try something like:
\usepackage{amssymb,amsmath,amsthm}
\usepackage[dutch]{babel}
\setlength{\parindent}{0cm}
\setlength{\parskip}{0cm}
\theoremstyle{break} % "break" style makes a linebreak after the header of "Definition X.X"
\newtheorem{defn}[]{Definitie} % you can put chapter or ...

Hi,

I/we haven't been using LaTeX for very long and I/we are getting a very peculiar error that to my knowledge should not happen:

! LaTeX Error: File `../projekt/preamble.tex' not found.
Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: tex)
Enter file name ...