See this from Apple on iCloud components not syncing:
If your iCloud Contacts, Calendars, or Reminders won’t sync - Apple Support
If those steps are ineffective, note that this issue can be caused by a 3rd party network filter, a network configuration mismatch or a corrupted service state specific to your Ethernet interface that occurred during the macOS Tahoe 26.6.2 update. Because the authentication handshake succeeds (verifying your password and prompting 2FA) but the finalized connection fails, the issue is not with your account or Apple’s servers—it is a local interface bug.
Check Third-Party Firewalls or Antivirus Content Filters
The macOS Tahoe 26.6.2 update introduced major kernel and security hardening patches backported from the macOS 27 beta. This frequently causes third-party security software (like Little Snitch, LuLu, Intego, or Avast) to block background system processes (like com.apple.iCloudHelper) specifically over hardware network lines while ignoring cellular paths.
- Turn off or temporarily uninstall any third-party security software or VPN configurations to test.
- Commercial VPNs claim to hide your browsing activity from websites, and while that may be true those VPN providers just sell your browsing data to others.
- Really, the only VPNs you should use are those provided by a school/employer to enable access to internal resources from a remote location. Any others should be uninstalled using the developer's instructions.
Delete and Recreate the Ethernet Service
Often, the network interface plist file gets bugged during major security point updates. Rebuilding the service forces macOS to map it cleanly.
- Open System Settings and go to Network.
- Click on your Ethernet connection.
- Click Delete Service.
- Next, in the main Network settings pans, click the ••• at the bottom then Add Service.
- Set the Interface to Ethernet, give it a name, and click Create.
- Click Apply to finalize the change and try signing into iCloud again.
Force Hardware MAC Address Refresh (Advanced Interface Toggle)
If deleting the service doesn't fix it, the Ethernet hardware profile might be retaining an explicit MTU or configuration state that blocks Apple's internal iCloud verification daemons.
- In System Settings > Network, select your newly recreated Ethernet service
- Click the Details... or Advanced button.
- Select the Hardware tab.
- Change the Configure dropdown from Automatically to Manually.
- Switch the MTU setting from Standard (1500) to Custom, and type in 1450 (this reduces packet fragmentation which can cause the timeout behavior you're seeing).
- Click OK and then Apply.
Check for Custom DNS or "Limit IP Address Tracking"
macOS Tahoe has strict built-in security features that handle iCloud data routing differently over high-bandwidth connections like Ethernet.
- In System Settings > Network > Ethernet, click Details...
- Look for Limit IP Address Tracking and toggle it Off. This feature masks your IP address for privacy but can trigger strict routing errors on local Ethernet switches post-update.
- Go to the DNS tab. If you see greyed-out router IPs, click the + button and temporarily add public DNS servers like Google's (8.8.8.8) or Cloudflare's (1.1.1.1) to ensure the Mac's background daemons aren't experiencing local routing loops.
