This error occurs when HomeKit is syncing things up with iCloud. To resolve it, follow these steps, as outlined by Apple:
- Check your Internet and Wi-Fi connections.
- Update your iOS device to the latest version of iOS.
- Restart your iOS device.
- Go to Settings > iCloud > iCloud Drive and make sure that your iOS device has iCloud Drive turned on.
- If you still have issues, turn off iCloud Keychain, then turn it back on. Make sure that you signed in to iCloud successfully.