I'm experiencing the same issue when "Force HTTPS" is enabled. However, when it's disabled, the SSL certificate generates without any problems.
Failed to issue a Let's Encrypt certificate for example.com: LetsEncrypt challenge failed for example.com: Some(ServerError { type: Some("urn:ietf:params:acme:error:connection"), title: None, status: Some(400), detail: Some("0.0.0.0: Fetching https://example.com/.well-known/acme-challenge/yC7zVeFjQRSqDuklVeHl-WuJS9CupU9YAiaoUfg3HX4: Connection refused") })