DAVx⁵ has been successfully tested with Zoho One.

Zoho One is a all-in-one suite to run your entire business. It includes CalDAV calendars and CardDAV contacts.

DAVx⁵ settings

CalDAV Base URL: https://calendar.zoho.com/ or https://calendar.zoho.eu/ (depending on where you have created the account)

CardDAV Base URL: https://contacts.zoho.com/ or https://contacts.zoho.eu/

User name and pasword: your Zoho email address and password. If you're using two-factor authentication, you have to generate and use an application-specific password for DAVx⁵.

Known problems

CardDAV: vCard contact groups are not supported. Choose the method used by your other clients or "per-contact categories" to minimize potential problems.

CardDAV: Contacts from the "Organization" address book are not provided via CardDAV and can't be synchronized.

CalDAV: VTODO tasks don't seem to be supported by Zoho.