DAVx⁵ has been successfully tested with Kopano.

Kopano is an open-source collaboration tool which also includes calendar, scheduling and address book functions.

DAVx⁵ Settings

Choose "Login with URL".

Base URL: https://your.server:8443/ or  http://your.server:8080 (see Kopano manual: Configuring CalDAV clients; you don't have to add /<user>/ etc.)

User name and password: your Kopano account

At the moment, Kopano doesn't support CardDAV, so contacts can't be synchronized.