https://whmcs.community/topic/215537-invalid-ip-response-connecting-to-api-using-ipv6-address/
Invalid IP response connecting to API using IPv6 address - Troubleshooting Issues - WHMCS.Community
I encountered a customer's installation that presented a new one for me. I updated the customer's system from 5.2.7 to 5.2.10 (incrementally, 5.2.8, 5.2.9...)...