You need to show the code that defines your Flask application "app".

Where is "app" defined? Are you importing it from another file?

Here is my attempt to reproduce the issue:

$ cat myproject.py
from flask import Flask

app = Flask(__name__)

@app.route("/")
def index():
    return "ok"

$ bin/gunicorn -w 4 -b 127.0.0.1:4000 myproject:app &
[1] 27435

2014-03-04 12:18:36 [27435] [INFO] Starting gunicorn 18.0
2014-03-04 12:18:36 [27435] [INFO] Listening at: http://127.0.0.1:4000 (27435)
2014-03-04 12:18:36 [27435] [INFO] Using worker: sync
2014-03-04 12:18:36 [27441] [INFO] Booting worker with pid: 27441
2014-03-04 12:18:36 [27442] [INFO] Booting worker with pid: 27442
2014-03-04 12:18:36 [27445] [INFO] Booting worker with pid: 27445
2014-03-04 12:18:36 [27448] [INFO] Booting worker with pid: 27448

$ curl http://127.0.0.1:4000/
ok

As you can see it works fine. You definitely don't need ProxyFix in this case.

You will need to downgrade werkzeug version from 1.0.0 to 0.16.0

This solved the problem for me.

Just run the following commands in your project:

python3 -m pip uninstall werkzeug

and then

python3 -m pip install werkzeug==0.16.0