Client takes almost one hour to create certificate

[jsha]

Possibly! I’d suggest filing an issue at https://github.com/letsencrypt/letsencrypt/. It probably doesn’t make sense to try and specifically detect IPv6 issues in the client, but there should probably be a timeout when making connections to the ACME server. The error message for those timeouts could suggest something like “please check the connectivity from your server to the outside Internet.”