Table of Contents does to build in PDF

Describe the bug

I am unable to build a TOC in the PDF Latex of my project. The TOC builds correctly on the HTML version. When I build the project on my local Win 10 computer everything works correctly, however when the project is build on my Linux based server, the TOC of the PDF does not build. Other projects build correctly on the server.

In the link to my project please scroll down to the bottom and select "download PDF" here you can see the fault.

How to Reproduce

$ git clone https://github.com/.../some_project
$ cd some_project
$ pip install -r requirements.txt
$ cd docs
$ make html SPHINXOPTS="-D language=de"
$ # open _build/html/index and see bla bla

In the link to my project please scroll down to the bottom and select "download PDF" here you can see the fault.

Expected behavior

No response

Your project

Please contact me and I can provide a copy of the project.

Screenshots

No response

OS

Win 10 and Linx

Python version

2.1.17

Sphinx version

unknown

Sphinx extensions

No response

Extra tools

No response

Additional context

No response

Start by obtaining the project referenced in the issue and reproducing the build on the Linux server using requirements.txt and the commands under docs. Compare the working HTML output in _build/html/index with the missing PDF table of contents; done means identifying and correcting the project-specific cause of the PDF-only failure.