Hi @HuTcHx.007
there is a new Letsencrypt certificate ( https://check-your-website.server-daten.de/?q=nextcloud-vosberg.duckdns.org ):
CN=nextcloud-vosberg.duckdns.org
03.03.2019
02.06.2019
expires in 89 days nextcloud-vosberg.duckdns.org - 1 entry
created Sunday.
So use that certificate two months. Then check, if the renew works.
If not, find your "DocumentRoot" in your Apache vHost and use that.
certbot run -a webroot -i apache -w yourDocumentRoot -d nextcloud-vosberg.duckdns.org -d www.nextcloud-vosberg.duckdns.org
PS: You have defined a www DNS A record with the same ip address. Then your certificate should have both domain names.