What does the CardDAV error message 'Principal not found' mean?

In short, this message typically means that the other side is likely not responding with a CardDAV server or not behaving according to the standard.

Common Causes

• Synology NAS: The address of a "QuickConnect ID" was used instead of the original NAS URL (Public DNS name/IP address + Port). Learn more here.
• The CardDAV server does not adhere to the RFC.