Hi @Uldrendan,
I’ve seen a slight increase in this error recently and I’m not really sure what could be causing it. Two relevant thoughts:
(1) What version of Certbot are you running? Is it recent?
(2) Is it possible that you have separate IPv6 and IPv4 server blocks anywhere in your configuration for the same hostname, rather than a single server block that listens in both IPv4 and IPv6?