Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
You look to be another sufferer of a Palo Alto Networks firewall. They changed their settings in recent months to block ACME challenge requests. You should talk with your network support and set an Application Rule to allow "acme challenge".
These examples might help them identify / test the setting
curl -I http://ipt.nsbm.ac.lk/.well-known/acme-challenge/SampleChallenge -A "Mozilla/5.0 (compatible; Let's Encrypt validation server; +https://www.letsencrypt.org)"
(fails with user-agent string that Let's Encrypt server uses)
curl: (56) Recv failure: Connection reset by peer
curl -I http://ipt.nsbm.ac.lk/.well-known/acme-challenge/SampleChallenge
(does not fail when using default curl user-agent string)
HTTP/1.1 301 Moved Permanently
Date: Sun, 26 Jun 2022 18:03:53 GMT
Content-Type: text/html; charset=iso-8859-1