I think the last thread on this is now locked, so I’m reposting.
Just went through an initial LetsEncrypt certgen on a fully updated 10.13.66-22 system. It failed certgen until I disabled the firewall on the Connectivity-Firewall page.
This is at least the fourth system in the last six weeks where this has been the case.
Something is decidedly hinky with the firewall and LetsEncrypt. If someone can give me instructions I will capture logs/data the next time this comes up.