@jlgm You are almost certainly affected by a Palo Alto brand firewall. You have the same symptoms as we often saw earlier in 2022. See (this link) for more info
You should talk to your network admins and have them change the Application Rule for "ACME protocol".
The tests described by my link above for your domain are this:
(test gets expected 404 with test file)
curl -I http://w3.dmat.ufrr.br/.well-known/acme-challenge/sometestfile
HTTP/1.1 404 Not Found
Server: nginx
(test fails when using a user-agent the same as Let's Encrypt servers)
curl -I http://w3.dmat.ufrr.br/.well-known/acme-challenge/sometestfile -A "Mozilla/5.0 (compatible; Let's Encrypt validation server; +https://www.letsencrypt.org)"
curl: (56) Recv failure: Connection reset by peer