Datapusher with gunicorn: WORKER TIMEOUT

I followed this official deployment guide:
http://docs.ckan.org/projects/datapusher/en/latest/deployment.html

But I faced this problem:

```
datapusher_1 | [2018-02-11 05:04:20 +0000] [525] [INFO] Booting worker with pid: 525
datapusher_1 | /usr/local/lib/python2.7/site-packages/ckanserviceprovider/web.py:13: ExtDeprecationWarning: Importing flask.ext.login is deprecated, use flask_login instead.
datapusher_1 | import flask.ext.login as flogin
datapusher_1 | [2018-02-11 05:04:50 +0000] [7] [CRITICAL] WORKER TIMEOUT (pid:525)
datapusher_1 | [2018-02-11 05:04:50 +0000] [525] [INFO] Worker exiting (pid: 525)
```

Any clue what might be causing it?