Problem solved!
OK. After the “fresh start” (see my previous post), I started to re-read all comments and I noticed, I haven’t really done everything that was suggested. My sincere apologies! I was very very busy at the time and not focused enough…
The key thing was to override the lines referring in the file/etc/httpd/conf.d/ssl.conf
by the Letsencrypt links:
For some reason it didn’t work to copy the certificates text into the contents of the files (perhaps I made a mistake), but overriding the links did work!
Thanks for this @sspencerwire . You pointed me to this! I just missed this key hint!
It proved not to be required to add the above Letsencrypt links in the httpd.conf, as the proper links are in the Included file /etc/httpd/conf.d/ssl.conf, so I removed them again from httpd.conf. It still works!
Also many thanks to @pajamian for pointing out how to test the certificate on a non-active server by putting the temporarily IP address of that server and the domain name in the /etc/hosts file:
Thanks all for you patience.
I learned a lot. Especially, to be focused and to READ carefully what you guys write!