Confirm the address book URL is https://dav.lightdrive.one/USERNAME/contacts/, with your LightDrive ID replacing USERNAME.
Use your full LightDrive email address as the username and either a current Contacts-only password created under Contacts → Mobile & app connections or a current Email & contacts password from Security settings. Your main Vault password, a Vault app password, and an Email-only password do not authenticate to CardDAV.
If the client has stored an old password, remove it and enter the correct newly created password when prompted. Confirm that the Contacts collection is enabled and that the sync app has contacts and background permissions.
Check which account owns the contact. A contact saved to a device-only, Google, Apple, Microsoft, or other address book is not sent to LightDrive.
Use the Vault Contacts app to confirm whether the server received a change. LightDrive Email reads CardDAV contacts but does not edit them; use the contacts synchronization control in Email to refresh its read-only copy.