Guys, thank you for the brainstorming session and all of your answers.
I solved this in the meantime.
As assumed there was an issue with the delegation of the DNS servers at my hoster.
They moved the domain to other auth NSs and since then the certbot successfully received certificates as desired.
My CNAME and ALIAS entries were correct and working perfectly fine now.
All have a great day and thanks again for your support.
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