Contacts failing to sync to iCloud from one of two Macs (boringssl_session_errorlog, ERROR: CGImageSourceCreateWithData: data is nil, -[__NSCFString rectValue]: unrecognized selector)
My main Mac (10.13.6) stopped syncing Contacts (but not Calendar or Notes) to iCloud a few weeks ago. A new sacrificial Mac (10.14.4) set up with a clean user account syncs fine, as do my iPhone and iPad. (I have been trying to avoid upgrading my main Mac to 10.14 because the iOS app management and Safari ad-blocking are much worse.)
This happened a few months ago; I nuked my mac’s Contacts and re-synced from iCloud, but the problem eventually returned.
Here are some censored highlights from the Console log for AddressBookSourceSync, from a single minute after during which I made a change to a contact on my main Mac. (There are 1069 AddressBookSourceSync entries during that minute.)