So, now I have created the certificate:
https://this-is-a-very-long-really-very-long-name-of-this-small-bdomain.server-daten.de/
The site is a standard site. But the subdomain is long enough.
So if you add a shorter domain name as CommonName, you can have such a long domain name.
PS: It's only a test certificate (Fake LE).
DNS-Name: www.server-daten.de
DNS-Name: this-is-a-very-long-really-very-long-name-of-this-small-bdomain.server-daten.de
DNS-Name: www.this-is-a-very-long-really-very-long-name-of-this-small-bdomain.server-daten.de
www has a standard redirect to non-www, http a standard-redirect to https.