Your failed http requests appear to be missing the “/pythonapp/” part of the URI. This can happen when you have some middleware or a proxy server adding a URL prefix that the Dash app is unaware of.
You might need to provide a value for the Dash constructor param ‘requests_pathname_prefix’. Maybe it wan’t to be “/pythonapp/”? But it’s been a while since I’ve had my head in that space and I might be getting it wrong.
This comment could be helpful: Deploy Dash on apache server [solved!]