I’m running an Open-Source CalDAV and CardDAV Server: Radicale
How can I conntect Nextcloud to this server?
Is it correct that Nextcloud will act as a server, too?
Or is there a client functionality in Nextcloud only?
(I don’t need 2 CalDAV and CardDAV Server in my LAN.)
Nextcloud has a build in server (SabreDAV) that can not be disabled as it also does all the webdav file things AFAIK. It has a client mode to allow it to sync with/subscribe to calendars on another CalDAV server, but then you are indeed running two CalDAV servers.
If I enable SabreDAV client mode, then I run only 1 server.
No. As far as I know (not an expert) is it only a way to inter-link two servers.
I would suggest you run two CalDav server for some time, while easily migrating to Nextcloud as you can subscribe to the ones on Radicale and after a while you can disable Radicale.