多人使用我的ajax工具导致内部服务器错误

Question

i'm still new to web developing but i made a little tool for me and my friends. it is supposed to send an ajax request every second to update a list of usernames of ppl who have signed up.

when i try it out alone, it works fine, but when i tried it out with 25 other people at the same time, everyone would get internal server errors...

here is the stripped down codesnippet:

function refreshLobby () {
  $.ajax({
    url: "core.php",
    type: "POST",
    data: {lobby: 1},
    success: function (output) {
      console.log(output);
      //irrelevant jquery html stuff
    }
  });
}

$(document).ready(function () {
  setInterval(function () {
    refreshLobby();
  }, 1000);
});

why is it working when i try it out alone, but starts bugging with internal server error when multiple people have it open at the same time ?