Thanks. Sorry for answering late. I got now shook up by an email I got from Let's Encrypt Expiry Bot yesterday, saying that my certificates are about to expire tomorrow.
Looking into /etc/letsencrypt/live/cert.pem I found that this cerificate is indeed expiring tomorrow.
But the certificate I'm actually using is
/var/lib/acme/.acme.sh/www.mydomain.org/fullchain.cer
which is due in August 9th.
So I'm wondering which process is still renewing the letsencrypt certificates in parallel to acme.sh?
How can I find out about that?
P.S. I also found out that I have already the desired extra domain (normalfilter.de) included in my certificate. So the question remaining is: "why or what is the cause that I'm still getting reminder email from Let's Encrypt Expiry Bot"?