Note on behalf of @ralf-agile-coast:
If you are using Mac as well as working with the compose stack of the full stack project template, and you are wondering why trying to connect to localhost:5000 gives you a 403 error right away:
After updating to Monterey you may have your Airplay-Receiver enabled by default and listening on :5000.
Disabling it does the trick: https://developer.apple.com/forums/thread/682332