err_empty_response error when clicking a button - php

Question

I have a website, and all its files (html/js/css/php) are on the same remote host. I have a button on the site, when clicked it sends a httprequest to a PHP file, waiting for a response.
My site works perfect almost everywhere, but there is one place which when I connect to the wi-fi, when clicking the button it return whit the error ERR_EMPTY_RESPONSE. Other buttons and PHP files work. It is only this specific button and in this specific wi-fi.
Does anyone know why does it happen and how to fix it?
details:
the button is <button class="btn btn-default col-sm-5" type="submit">
the onClick function:
function search() {
var param = "";
var firstParam = true;
var form = document.getElementById("formCS");
var name = document.getElementById("songName").value.trim();
if (name != "") {
firstParam = false;
param += "name=" + name;
}
var creator = document.getElementById("creator").value.trim();
if (creator != "") {
if (firstParam) {
param += "creator=" + creator;
firstParam = false;
} else {
param += "&creator=" + creator;
}
}
var type;
if (document.getElementById("partners").checked) {
type = "partners";
} else if (document.getElementById("circle").checked) {
type = "circle";
} else if (document.getElementById("lines").checked) {
type = "lines";
} else if (document.getElementById("none").checked) {
type = "";
}
if (type != "") {
if (firstParam) {
param += "type=" + type;
firstParam = false;
} else {
param += "&type=" + type;
}
}
var year = document.getElementById("year").value;
if (year != "none") {
if (firstParam) {
param += "year=" + year;
} else {
param += "&year=" + year;
}
}
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function () {
if (xhttp.readyState == 4 && xhttp.status == 200) {
handleResponse(xhttp.response);
}
};
xhttp.open("GET", "search.php?" + param, true);
xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xhttp.send();
}
the search.php file connects to a DB on the remote host and runs a sql query. nothing different from other buttons on the site.
the wi-fi is a free wi-fi provided by an academic institue.
after clicking the button there is an error in the console:
GET http://rikudim.info/search.php? net::ERR_EMPTY_REPONSE