Can't get all parts of burp-ui playing nice together

Hello,

I installed latest burp-ui some days ago using these instructions http://burp-ui.readthedocs.io/en/latest/development.html I get everything to run, but I can't create new clients in the GUI nor update server settings. I always get "Internal server error". Using burp-ui with gunicorn, I checked the logs, they are empty. Launching the agent with -vvvv doesn't give any errors either. I then modified /etc/burp/burpui_gunicorn.py by setting loglevel to debug, but I don't see any errors:

[2018-03-07 14:02:33 +0100] [17573] [DEBUG] PUT /api/settings/agent1/config
[2018-03-07 14:02:35 +0100] [17573] [DEBUG] GET /api/clients/agent1/backup-running
[2018-03-07 14:02:40 +0100] [17570] [DEBUG] Closing connection.

I tried to launch burp-ui -c /etc/burp/burpui.cfg directly, but then I happen to get this:

Usage: flask run [OPTIONS]

Error: The file/path provided (burpui.cli) does not appear to exist.  Please verify the path is correct.  If app is not on PYTHONPATH, ensure the extension is .py

When launching bui-manage -c /etc/burp/burpui.cfg I get

Usage: flask [OPTIONS] COMMAND [ARGS]...
...

For the sake of sanity, I reinstalled burp-ui as shown in the development instructions right now, but I get the same. I am totally unable to find any error logs when using gunicorn, and can't launch burp-ui manually to fetch them.

Could you please give me some advice here ?

Best regards.