“Let’s Encrypt™” HTTP DCV error: Timeout during connect (likely firewall problem)

Symptoms

You encounter an error similar to the following when running AutoSSL.

11:41:18 PM WARN “Let’s Encrypt™” HTTP DCV error (domain.tld: 400 urn:ietf:params:acme:error:connection (The server could not connect to a validation target) (203.0.113.2: Fetching http://domain.tld/.well-known/acme-challenge/3RJ3W8AGIGxAjBXJQcQGKD-8k-JQF3p9hmzWuFKyZds: Timeout during connect (likely firewall problem))

Description

The error occurs when Let's Encrypt is unable to reach the site on TCP port 80 for the HTTP DCV check.

Workaround

Ensure that inbound TCP port 80 is open for all locations through all relevant network and security devices and applications.