DAVx⁵ has been successfully tested with luckycloud.

luckycloud is an encrypted sync and share platform for the highest level of security and performance.

DAVx⁵ Settings

Choose "Login with URL".

Base URL:

User name and password: your luckycloud account and password

Contact group method: Luckycloud uses VLIST vCards for contact groups, which is not specified in CardDAV and thus not supported by DAVx⁵.