After a fresh installation I get the following warnings:
What do I have to do?
That’s… a lot of warnings… and sadly I don’t speak German (well, not much). Not all of these are going to be vital (such as upgrading PHP, although it will make Nextcloud faster).
Given that the errors are mainly in German, you may do better to ask in the German sub forum…
It would be easier if you give more information could. how do you install. where did you have nextcloud (maybe webhoster?)
A lot of security / hardening issues. Solutions described here:
Geht alles in Richtung Sicherheits / Webserverhärtung. Anleitung hier:
https://doc.owncloud.org/server/8.0/admin_manual/configuration_server/harden_server.html