Contacts not syncing with iCloud

Since at least a couple of weeks, the contacts on my iMac have not being syncing with iClouds, while all my other devices (iPhone, iPad, MBP) do. Not sure whether that's relevant, but the iMac is running HighSierra while the MBP is still running Sierra.

I tried all obvious things, e.g. rebooting the iMac, turning off and on Contacts on System Preferences/Internet Accounts/iCloud, disabling/reenabling the iCloud account on the Contact app, but nothing has worked so far.