After dis-installing and completely removing nginx and acme.sh, re-installing them again from scratch, and retrying to issue certificates, and getting the same “Connection refused” without any apparent reasons, and this for two times, I decided to try with Certbot.
I followed the well done indications found here: https://gist.github.com/cecilemuller/a26737699a7e70a7093d4dc115915de8 and got the certificates, which, according to https://www.ssllabs.com/ssltest/analyze.html?d=www.ggc.world has an overall rating of A.
But with https://check-your-website.server-daten.de/?q=ggc.world I get this error:
|C| Error - no version with Http-Status 200
|H| fatal error: No https - result with http-status 200, no encryption
For this new error type I opened a new Help Request: Https://check-your-website.server-daten.de/?q=ggc.world : fatal error: No https - result with http-status 200, no encryption