Troubles with undefined citations

Question

I am new to Latex and having trouble with undefined citations. I am using Texmaker on Ubuntu.

I have tried all of the suggestions so far, but the same problem persists.

This is most my main .tex-file that I use for quick build:

\documentclass[12pt]{book}

\usepackage{a4wide}
\usepackage{fancyhdr}
\usepackage{graphicx}
\usepackage[english, ngerman]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[bookmarks]{hyperref}
\usepackage{braket}
\usepackage{mathrsfs}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{natbib}

\pagestyle{fancyplain}
\renewcommand{\chaptermark}[1]%
         {\markboth{\thechapter.\ #1}{}}
\renewcommand{\sectionmark}[1]%
         {\markright{\thesection\ #1}}
\lhead[\fancyplain{}{\bfseries\thepage}]%
    {\fancyplain{}{\bfseries\rightmark}}
\rhead[\fancyplain{}{\bfseries\leftmark}]%
    {\fancyplain{}{\bfseries\thepage}}
\cfoot{}

\hypersetup {
pdfpagemode = {UseNone}
}

\begin{document}


  \frontmatter


  \include{pages/deckblatt}


  \include{pages/titleblatt}


  \include{pages/zweite_seite}


  \tableofcontents
  \markboth{Inhaltsverzeichnis}{Inhaltsverzeichnis}


  \listoffigures
  \markboth{Abbildungsverzeichnis}{Abbildungsverzeichnis}


  \listoftables
  \markboth{Tabellenverzeichnis}{Tabellenverzeichnis}
  \cleardoublepage


  \markboth{Zusammenfassung}{Zusammenfassung}
  \include{pages/zusammenfassung}


  \mainmatter\setcounter{page}{1}
  \include{chapter/kap_01}
  \include{chapter/kap_02}
  %\include{chapter/kap_03}
  %\include{chapter/kap_04} usw.
  \include{pages/anhang}


  \backmatter
  \bibliographystyle{unsrtnat}
  \bibliography{literatur}
  \markboth{}{}


  \include{pages/danksagung}


\end{document}

And this is the literatur.bib file in the same folder:

@Article{EPR,
  author    = {A. Einstein and B. Podolsky and N. Rosen},
  title     = {Can quantum-mechanical description of physical reality be considered complete?},
  journal   = {Physical Review},
  publisher = {American Physical Society},
  year      = {1935},
  volume    = {47},
  pages     = {777-780}
}

@Article{Aspect,
  author    = {A. Aspect and J. Dalibard and G. Roger},
  title     = {Experimental Test of Bell's Inequalities Using Time-Varying Analyzers},
  journal   = {Physical Review Letters},
  publisher = {American Physical Society},
  year      = {1982},
  volume    = {49},
  number    = {25},
  pages     = {1804-1807}
}

@Article{Bell,
  author    = {J. S. Bell},
  title     = {On the Einstein Podolsky Rosen Paradox},
  journal   = {Physics},
  publisher = {Physics Publishing Co.},
  year      = {1964},
  volume    = {1},
  number    = {3},
  pages     = {195-200}
}

@Article{Stern-Gerlach,
  author    = {W. Gerlach and O. Stern},
  title     = {Der experimentelle Nachweis der Richtungsquantelung im Magnetfeld},
  journal   = {Zeitschrift für Physik},
  band      = {9},
  year      = {1922},
  pages     = {349-352}
}

@Article{Bertlmann,
  author    = {J. S. Bell},
  title     = {Bertlmann's socks and the nature of reality},
  journal   = {Journal de Physique Colloques},
  year      = {1981},
  volume    = {42},
  number    = {C2},
  pages     = {41-62}
}

@Article{Bohm,
  author    = {D. Bohm and H. Aharonov},
  title     = {Discussion of experimental proof of the paradox of Einstein, Rosen, and Podolsky},
  journal   = {Physical Review},
  publisher = {American Physical Society},
  year      = {1957},
  volume    = {108},
  number    = {4},
  pages     = {1070-1076}
}

@Article{Innsbruck,
  author    = {G. Weihs and T. Jennewein and C. Simon and H. Weinfurter and A. Zeilinger},
  title     = {Violation of Bell’s inequality under strict Einstein locality conditions},
  journal   = {Physical Review Letters},
  publisher = {American Physical Society},
  year      = {1998},
  volume    = {81},
  number    = {23},
  pages     = {5039-5043}
}

@Article{CHSH,
  author    = {J. Clauser and M. Horne and A. Shimony and R. Holt},
  title     = {Proposed experiment to test local hidden-variables theories},
  journal   = {Physical Review Letters},
  publisher = {American Physical Society},
  year      = {1969},
  volume    = {23},
  number    = {15},
  pages     = {880-884}
}

@Article{Loopholefreebelltest,
  author    = {B. Hensen et al.},
  title     = {Loophole-free Bell test using electron spins in diamond: second experiment and additional analysis},
  journal   = {Scientific Reports},
  publisher = {Nature},
  year      = {2016},
  volume    = {6},
  number    = {30289},
  pages     = {1-11}
}

@Book{Ballentine,
    author        = {L. Ballentine},
    booktitle     = {Quantum Mechanics: A Modern Development},
    publisher     = {World Scientific Publishing Company},
    edition       = {2nd revised and edited edition},
    year          = {2014}
}

@Book{Sakurai,
    author        = {J. Sakurai and J. Napolitano},
    booktitle     = {Modern Quantum Mechanics},
    publisher     = {Pearson Education (US)},
    edition       = {2nd edition},
    year          = {2010}
}

@incollection{GHZ,
    author    = {D. Greenberger and M. Horne and A. Zeilinger},
    title     = {Bell’s Theorem, Quantum Theory and Conceptions of the Universe},
    series    = {Fundamental Theories of Physics},
    volume    = {37},
    publisher = {Springer},
    year      = {1989},
    pages     = {69-72}
}

@Book{Cohen-Tannoudji-1,
    author        = {C. Cohen-Tannoudji and B. Diu and F. Laloë and J. Streubel},
    booktitle     = {Quantenmechanik},
    volume        = {1},
    publisher     = {De Gruyter Studium},
    edition       = {5. Auflage},
    year          = {2019}
}

@Book{Cohen-Tannoudji-2,
    author        = {C. Cohen-Tannoudji and B. Diu and F. Lalo{\"e} and J. Streubel},
    booktitle     = {Quantenmechanik},
    volume        = {2},
    publisher     = {De Gruyter Studium},
    edition       = {5. Auflage},
    year          = {2019}
}

@Book{Bub,
    author        = {J. Bub},
    booktitle     = {Interpreting the Quantum World},
    publisher     = {Cambridge University Press},
    edition       = {1st revised edition},
    year          = {1999}
}

When I want to insert the citation in the appropriate place in kap_01.tex, I start typing \cit..., and I am already suggested \citep{According names in the .bib file}, which makes me think that the .bib file is in the right place and recognized, which seems to be the reason for most similar issues. There is some peculiar behavior here though: Sometimes the bib entries are not suggested and then, without me being aware of having made any changes, after a restart of the program, the suggestions are there again...

When I go "quick build", it reports undefined citations wherever I have used them and they are shown as question marks in the pdf.

Unfortunately, the log is to long to attach here, instead I've uploaded the file here: https://easyupload.io/9jhap8

The only .blg file I could find was last modified in Ocotber, so that can't be it. Back then I hadn't edited the bib-file either.