supervisord http://localhost:9001 refused connection

Despite much effort trying all solutions posted on stackoverflow, I still cannot manage solving this.

The problem (Case 1):

$ sudo supervisorctl -c /app/vpn_bot/supervisord.conf
http://localhost:9001 refused connection

Case 2:

$ sudo supervisorctl -c /app/vpn_bot/supervisord.conf
unix:///tmp/supervisorctl.sock refused connection

Here is the relevant supervisord.conf file:

[supervisord]
# nodaemon=true
[supervisorctl]
# case 1: serverurl=http://127.0.0.1:9001
serverurl=unix:///tmp/supervisorctl.sock # case 2

[unix_http_server]
file=/tmp/supervisorctl.sock

[inet_http_server]
port=127.0.0.1:9001

[rpcinterface:supervisor]
supervisor.rpcinterface_factory = supervisor.rpcinterface:make_main_rpcinterface

I have made sure sudo supervisord -c /app/vpn_bot/supervisord.conf is running, and port 9001 is not used by any other process.

Any one can offer some help here?