DAVx⁵ has been successfully tested with Dolibarr.

Dolibarr is an Open Source ERP & CRM for business. It has CalDAV/CardDAV support with the CDav module.

DAVx⁵ settings

Base URL:

User name: your Dolibarr email address
Password: your Dolibarr password

Be sure to have at least one event in each calendar, otherwise the server does not offer the calendar to DAVx5.