You've hit the Let's Encrypt rate limit because too many authorization attempts failed recently. You get a maximum of 5 per hour. Rate Limits - Let's Encrypt
The main problem will be that when the challenge response is written to disk, the webserver responding on port 80 must reply with these challenge responses (from disk). so http://homeassistant***.duckdns.org/.well-known/acme-challenge/<a response file> has to work, or http validation will just never work.
How is nginx involved here, is it a reverse proxy back to home assistant? Hassio has a Let's Encrypt plugin you can use.