Before trying to set-up sync: Enable CalDAV/CardDAV for your clients in Settings → Synchronization (see Group Office wiki).
For CalDAV use:
For CardDAV use:
Contact group method: No CardDAV contact group support. Choose "per-contact categories" as contact group method to minimize problems.
To use both CalDAV and CardDAV with a single DAVx⁵ account, configure well-known URLs (recommended).