net::ERR_CONNECTION_REFUSED . How to correctly configure Nginx with Socket.io?

[MarcoI]

Hi Francis,
I "solved" this problem installing the Desktop version of Ubuntu 18.04 as I
described here: 
https://askubuntu.com/questions/1207812/webapp-fails-with-neterr-connection-refused-with-ubuntu-18-04-4-server-edition

Now I've got a different, but may be, similar, problem, which I described in
this post in the Nginx Forum:
https://forum.nginx.org/read.php?2,286991

Marco

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,286850,286993#msg-286993