After upgrade to 4.0.1, page showing 502 Bad Gateway

Before upgrade everything was fine.

Followed the guide in https://github.com/iNextrix/ASTPP/blob/v4.0.1/doc/Installation/upgrade.rst

Then open the main page https://1.2.3.4/ it is showing 502 Bad Gateway

in the log /var/log/nginx/astpp_error.log
2019/08/14 06:46:12 [error] 18287#18287: *10 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: 7.8.9.0, server: _, request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/var/run/php/php7.3-fpm.sock:", host: "1.2.3.4"

What might be the problem? and where should I check please?

Thanks.