You may have used tls-sni-validation (via port 443).
But tls-sni-validation is deprecated, support ends 2019-02-13.
So Certbot switches to http - validation.
Then an open port 80 + http is required.
Your port 80 is closed or doesn't answer.
The tool ( https://check-your-website.server-daten.de/?q=elberthome.dnshome.de ) should see a http status 404 - not found.
But not a timeout.