[macOS] Lets Encrypt (production) cannot obtain certificate after validating challenge

[TCFox]

Seems to be an issue only on macOS, and only with the production server (i.e. not Let's Encrypt Staging or with the Function running on Windows).

The socket gets closed on the HttpClient underlying Certes. I suspect it's a TLS thing, but it is strange that this happens after the challenge is validated (at the certificate issuance stage).