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 a LightDrive client password that includes Contacts. This may be a Contacts only, Email & contacts, Contacts & calendars, or Email, contacts & calendars password created under Settings → Security → App & device access.
Your main LightDrive password, Vault app passwords, and client passwords that do not include Contacts do not authenticate to CardDAV.
If the client has stored an old password, remove it and enter the correct client 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.