This was happening only on 1 of my machines, so bit the bullet and reinstalled from scratch. Problem cured, so don't know specific cause.

I have this problem, too. But in both directions synchronization does not work.

As stated above. Had to completely remove (used RevoUnistall) and the reinstalled from scratch.

People who have Revo uninstaller almost always also have a Third party antivirus solution and they are perhaps the single greatest source of network failure (unless you only use a browser and web pages). When sync of any sort fails, my first port of call looking to a cause is to uninstall any antivirus other than Windows Defender, stop any VPN and check the relevant folder are not included in some form of streaming backup or cloud drive synchronization. Or restart the device in safe mode with networking if it has a wired internet connection as that excludes all of the above startup stuff.

Thanks for your help.

Meanwhile I could solve the synchronization issue on my machine by doing the following steps:

1. Goto Adressbooks and remove iCloud contacts
2. Goto Settings, scroll down, open config editor
3. Show only modified preferences and search for "carddav"
4. Delete all preferences that contain "carddav" (In my case, this is only iCloud)
5. Restart TB
6. Add iCloud contacts in Adressbooks with new app password