Hi Frank, the app thinks it has updated your DNS server OK. It appears as if the DNS server Certify is updating is not your public DNS server that Let’s Encrypt will see.
Can you confirm that the authoritative DNS nameserver you expect to update are ns1.domainknecht.de and ns2.domainknecht.de.
You can check these during the request to see if the TXT record is present using dig -t TXT _acme-challenge.mail.apparet-it.net from a linux (or WSL) terminal or from a windows command prompt nslookup -type=txt _acme-challenge.mail.apparet-it.net