I recently tried to obtain certificates and to renew certificated for 2 domains hosted on 2 different servers and receive the same error
FailedChallenges: Failed authorization procedure… (http-01): urn:acme:error:unauthorized :: The client lacks sufficient authorization :: Invalid response from http…
But on both servers files exist in .well-known/acme-challenge
If I try to access them in browser, I succeed.
DNS records for domains are quite old, dig resolves them.
What is more - there is nothing in nginx access and error logs except my own IP, seems that letsencrypt did not queried my domains at all.
3 months ago everything was fine with these servers and domains.