SYntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

function loadContent(root, suffix, currentPage) {
 
    let objects = JSON.parse(Get(root + "/rest/players" + suffix).responseText);
    let playersCount = Get(root + "/rest/players/count" + suffix).responseText;
    document.getElementById("count").innerText = "Players found: " + playersCount;
    let table = document.getElementById("mainTable");
    table.innerHTML = "";
    createPaging(document.getElementById("limit").value, playersCount, currentPage);
 
    for (let i = 0; i < objects.length; i++) {
 
        let tr = document.createElement("tr");
        let th = document.createElement("th");
        th.setAttribute("scope", "row");
        th.appendChild(document.createTextNode(objects[i].id));
        tr.appendChild(th);
        let td1 = document.createElement("td");
        td1.appendChild(document.createTextNode(objects[i].name));
        tr.appendChild(td1);
        let td2 = document.createElement("td");
        td2.appendChild(document.createTextNode(objects[i].title));
        tr.appendChild(td2);
        let td3 = document.createElement("td");
        let race = objects[i].race.charAt(0).toUpperCase() + objects[i].race.slice(1).toLowerCase();
        td3.appendChild(document.createTextNode(race));
        tr.appendChild(td3);
        let td4 = document.createElement("td");
        let profession = objects[i].profession.charAt(0).toUpperCase() + objects[i].profession.slice(1).toLowerCase();
        td4.appendChild(document.createTextNode(profession));
        tr.appendChild(td4);
        let td5 = document.createElement("td");
        td5.appendChild(document.createTextNode(objects[i].experience));
        tr.appendChild(td5);
        let td6 = document.createElement("td");
        td6.appendChild(document.createTextNode(objects[i].level));
        tr.appendChild(td6);
        let td7 = document.createElement("td");
        td7.appendChild(document.createTextNode(objects[i].untilNextLevel));
        tr.appendChild(td7);
        let td8 = document.createElement("td");
        let birthday = new Date();
        birthday.setTime(objects[i].birthday);
        td8.appendChild(document.createTextNode(birthday.toDateString()));
        tr.appendChild(td8);
        let banned;
        if (objects[i].banned) {
            banned = "banned";
        } else {
            banned = "active";
        }
        let td9 = document.createElement("td");
        td9.appendChild(document.createTextNode(banned));
        tr.appendChild(td9);
        let warButton = document.createElement("button");
        warButton.setAttribute("type", "button");
        warButton.setAttribute("class", "btn btn-warning btn-sm");
        warButton.appendChild(document.createTextNode("Edit"));
        warButton.addEventListener("click", function () {
            editButtonClick(root, tr, objects[i].id)
        });
        let dangerButton = document.createElement("button");
        dangerButton.setAttribute("type", "button");
        dangerButton.setAttribute("class", "btn btn-danger btn-sm");
        dangerButton.appendChild(document.createTextNode("Delete"));
        dangerButton.addEventListener("click", function () {
            processDelete(root, objects[i].id)
        });
        let td10 = document.createElement("td");
        td10.appendChild(warButton);
        tr.appendChild(td10);
        let td11 = document.createElement("td");
        td11.appendChild(dangerButton);
        tr.appendChild(td11);
        table.appendChild(tr);
    }
    window.scrollTo(500, 100);
}
 
function Get(requestUrl) {
    let Httpreq = new XMLHttpRequest(); // a new request
    Httpreq.open("GET", requestUrl, false);
    Httpreq.send(null);
    if (Httpreq.status === 400) {
        $('#error-text').text("Bad request to GET " + requestUrl);
        $('#myModal').modal('show');
    }
    if (Httpreq.status === 404) {
        $('#error-text').text("Not found GET " + requestUrl);
        $('#myModal').modal('show');
    }
    return Httpreq;
}