Download of notebooks as PDF is running into 500-Internal Server Error

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Hello, the following issue describes the PDF via LaTeX Export of a notebook from jupyter which is running into an error.

**Describe the bug**
PDF via LaTeX Export of a notebook is running into 500-Internal Server Error.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a simple Notebook
2. Fill a row with "print('hello')" out
3. Click on File -> Download as... -> PDF via LaTeX
4. See error (attaached to this issue)

**Expected behavior**
- Working PDF Export

**Screenshots**
see attached error message
[PDF-Export-Error.txt](https://github.com/jupyter/notebook/files/8177104/PDF-Export-Error.txt)

**Desktop**
- OS: any OS and any OS version
- Any browser and any browser versions

**Server Side**
- OS: Red Hat Enterprise Linux Server
- OS Version: 7.9 (Maipo)
- Patchlevel: latets
- Notebook server version: 6.0.3
- Python version: 3.7.6

Deeper investigation unfortunatly not possible. Thank you in advance for any advice and hint in resolution of that issue

Start by reproducing the File → Download as... → PDF via LaTeX workflow described in the issue, using the attached PDF-Export-Error.txt as the initial error evidence. Trace the server-side export failure and verify that the simple notebook produces a working PDF instead of a 500 error.

Written by the indexing model from the issue text.