mailbox.org is a German email provider and focused on privacy. mailbox.org address books, calendars and tasks can be synchronized with DAVx⁵.
Choose "Login with email address" and enter your mailbox.org email address and password.
If you use your own domain with mailbox.org you have to use the "Login with Base URL" option. Enter your email address as user name and your password.
Contact group method: There is no server-side CardDAV contact group support. Choose "per-contact categories" as contact group method to minimize problems.