BibTeX: warning: 1 characters of junk seen at toplevel

Question

I have googled for hours without finding any solution for my warnings. The warning says: BibTeX: warning: 1 characters of junk seen at toplevel. I have 21 working Citations in my bibliography. If i add one more, doesn't matter which one, ill get the warning (even a copied one with a eg. changed name)

@article{courbariaux2016binarized,
title={Binarized Neural Networks: Training Deep Neural Networks with Weights and Activations Constrained to +1 or -1},
author={Matthieu Courbariaux and Itay Hubara and Daniel Soudry and Ran El-Yaniv and Yoshua Bengio},
year={2016},
eprint={1602.02830},
archivePrefix={arXiv},
primaryClass={cs.LG}
}

It does not matter if I use another bibtex citation that worked already. I get the Warning for every Bibtex file. Not sure if you have enough Information to help me.

I use Natbib and Overleaf.

\documentclass[a4paper,12pt,twoside, numbers=noenddot]{scrartcl} 
\usepackage[backend=biber,natbib=true]{biblatex}
\addbibresource{literatur/bibliography.bib} 
% }
% @book{meyer2021embedded,
% title={Embedded Microprocessor System Design Using FPGAs},
% author={Meyer-Baese, Uwe},
% year={2021},
% publisher={Springer Nature},
% pages ={1}
% } 
\begin{document} 
\cite{meyer2021embedded}
\newpage
\end{document}

Answer 1

The warning is from biber (not bibtex) and generated if there are characters in the bib file that are not in an @xxx{...} entry.

For example if I take the commented entry from your tex file and save as bibliography.bib

with content

}
@book{meyer2021embedded,
title={Embedded Microprocessor System Design Using FPGAs},
author={Meyer-Baese, Uwe},
year={2021},
publisher={Springer Nature},
pages ={1}
}

Then biber will generate the output

INFO - Looking for bibtex format file 'bibliography.bib' for section 0
INFO - LaTeX decoding ...
INFO - Found BibTeX data source 'bibliography.bib'
WARN - BibTeX subsystem: /tmp/biber_tmp_ludL/f4d088b3f9f145b5c3058da33afd57d4_750.utf8, line 2, warning: 1 characters of junk seen at toplevel

This is due to the spurious } at the start of the file. Deleting it makes the warning go:

@book{meyer2021embedded,
title={Embedded Microprocessor System Design Using FPGAs},
author={Meyer-Baese, Uwe},
year={2021},
publisher={Springer Nature},
pages ={1}
} 

Note that the warning does not say what the bad characters are but does give the line number (or apparently one more than the line number) at the point the error is detected so the } on line 1 generated

line 2, warning: 1
^^^^^^^

So you should be able to find the bad characters quite easily.