Apple uses idmsa.apple.com (which resolves to idmsa.apple.com.akadns.net) for authentication. Currently (Feb 8, 5:30 PM BST) it's unreachable: https://viewdns.info/chinesefirewall/?domain=idmsa.apple.com.akadns.net
Tell HN: iCloud sign-in is blocked in China | Heykuki News