Unfortunately, this syntax was broken in the latest version of gunicorn (or never officially supported in the first place) - potential regression: failed to parse WSGI callable as attribute of object · Issue #2213 · benoitc/gunicorn · GitHub
So, now you have to create a variable named server inside app.py rather than referring to it as app.server. That is, include:
app = dash.Dash(__name__)
server = app.server