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hello andre. this is a little bit strange because I can't reproduce your error. Your given minimal working example compiles on my system without any errors. Maybe you have to configure your texmaker to use biber and not bibtex or something else for the compile step. I'm not to familiar with texmaker ...

it is "tudelft-report.cls". This seems to be a custom document class of yours so I can't say where you put it. Use your search function of the file explorer if you don't remember.

Sry Nepo, I'm not to familiar with bibtex (basicly never used anything else then biblatex). You would have to wait for someone else to help you with this. But since this is a pretty basic question, you could look into the manual for the bibtex-package in the mean time. Maybe you will find the answer ...

sry, I'm not able to answer your follow-up question. Maybe someone else will be.

i'm sry, but it seems this isn't possible in your document class of choice. You could try a different one like for example the newer "scrlttr2". In this you could at least make a subject with:

Your code is not compilable. There are definitely many errors, not just one or two. Please post a minimal working example.

I do not understand your problem.

Hi Nepo! This is no big deal. You just have to use the "maxnames"-attribute when calling biblatex and give it at least the value of the number of names in your citation with the biggest number of names. For example:
\usepackage[maxnames=7]{biblatex}
You could also set it to a very high number to ...

I think the problem lies in Try deleting that and instead you may use \tableofcontents[currentsection] later on in you document.

which bibliography package do you use?
which bibliography style do you use?