HTTP Open Proxies
An open proxy allows any Internet user to send or receive messages using that server to hide the sender. Sometimes it can be used to provide privacy for users and bypass censorship, but often times it is unintentional. Your server could be abused to send spam because of a misconfiguration of your server.
IRC Networks will often check to see if your IP address is running an HTTP open proxy. If they discover it, they may ban your entire IP address:
17:55 -!- usename [email@example.com] has quit [K-banned: [exp/proxy] Open proxies are not allowed on DALnet. See http://kline.dal.net/proxy/proxyinfo.php?proxy=HTTPPOST&ip=192.168.0.1&port=8080 for more information. (2021/01/29 09.55)]
Normally this should not occur if you have properly configured openhttpd and have not installed any insecure web applications or open web proxies.
First, double check to make sure there are no insecure web applications or open web proxies.
Secondly, make sure your firewall bans any packets to incoming ports except those that have been whitelisted by your firewall. For example, blocking port 8080 here would have prevented the proxy scanner from reporting an open proxy.