flask.cli.NoAppException: Application crashing

This error may be a sign that an import is not found. To see which import is missing, try to run your server.py file with the python interpreter:

    python yourapp.py

Example of output :

    Traceback (most recent call last):
        File "yourapp.py", line 4, in <module>
         from flask_httpauth import HTTPBasicAuth
    ImportError: No module named flask_httpauth

Source (and other leads)

The issue was that some package were missing or corrupted.

I reinstalled everything with pip3 install -r requirements.txt --ignore-installed and now it works fine.