This looks like a reputation based firewall blocking requests. It would not affect just Let's Encrypt. I can't query your "home" page using HTTP from my own test server in the USA
curl -i http://minterrors.org/
HTTP/1.1 403 Forbidden
<HTML><HEAD><TITLE>403 Forbidden</TITLE></HEAD><BODY><center>
<b>Threat Prevention</b></center>
<p>This site is blocked because it violates network policy.</p>
<p>Host: minterrors.org</p>
<p>URI: /</p>
<p>Reason: Threat reputation No reputation</p>
<p>Please contact your network administrator</p></BODY></HTML>
Can you make a rule that allows any URI that starts with /.well-known/acme-challenge