Enable debug for Apple Contacts

For CardDAV logs, in Terminal type:

defaults write -g ABMigrationLogEnabled -bool YES
defaults write com.apple.AddressBook.CardDAVPlugin EnableDebug -bool YES
defaults write com.apple.AddressBook.CardDAVPlugin LogConnectionDetails -bool YES

Then relaunch Contacts and see if you can reproduce the problem. If so, then attach the following: ~/Library/Containers/com.apple.AddressBook/Data/Library/Logs/CardDAVPlugin

Then disable the logging by typing in Terminal again:

defaults delete -g ABMigrationLogEnabled
defaults delete com.apple.AddressBook.CardDAVPlugin EnableDebug
defaults delete com.apple.AddressBook.CardDAVPlugin LogConnectionDetails