The domain www.flystore.net is being served through Cloudflare CDN. Any Let's Encrypt certificate installed on the origin server will only encrypt traffic between the server and Cloudflare. It is strongly recommended that the SSL option 'Full SSL (strict)' be enabled.
https://support.cloudflare.com/hc/en-us/articles/200170416-What-do-the-SSL-options-mean-
Challenge update failures for www.flystore.net in order https://acme-staging-v02.api.letsencrypt.org/acme/order/5751349/4462274984
acme: error code 403 "urn:ietf:params:acme:error:unauthorized": Cannot negotiate ALPN protocol "acme-tls/1" for tls-alpn-01 challenge