web application firewall
Web application firewalls are software that inspects HTTP requests and tries to block malicious or suspect requests, often with a high risk of false positives due to broad defaults, and limited effectiveness against a motivated attacker.
An example is mod_security
If you are on shared hosting, your hoster might have one enabled by default, so be on the lookout for its reports in server logs when requests are failing without an obvious reason. It should log that it blocked a request and why it did so.