Thank you for your response!
I've found the issue.
It is the incorrect .well-known redirects of Apple Mac OS.
The fix is here: [Solved] Apple server proxy blocks access to .well-known - #6 by cyrilpic
The cleanest solution I found was to create a new custom proxy site for apples apache serviceproxy:
/Library/Server/Web/Config/Proxy/apache_serviceproxy_customsites_letsencrypt.conf:
ProxyPass /.well-known/acme-challenge http://127.0.0.1:34543/.well-known/acme-challenge
ProxyPassReverse /.well-known/acme-challenge http://127.0.0.1:34543/.well-known/acme-challengeThat file is automatically loaded by the proxy thru the file /Library/Server/Web/Config/Proxy/apache_serviceproxy.conf
After I added the file I restarted the proxy thru launchctl:
sudo launchctl stop com.apple.serviceproxy because the proxy itself doesn't restart when the web service is restarted.
Thank to DDJarod