Ok, I have looked over everything so many times and this doesn't work. I'm going crazy. The code:

function _(element) {
        return document.getElementById(element)
    }

    var label = _("label_chat")
    label.addEventListener("click", function(){
        var inner_pannel = _("inner_left_pannel")

        var ajax = new XMLHttpRequest()
        ajax.onload = function() {
            if(ajax.status == 200 && ajax.readyState == 4) {
                inner_pannel.innerHTML = ajax.responseText
            } else {
                alert("ERROR")
            }
        }

        ajax.open("POST","file.php",true)
        ajax.send()
    })

There is a file.php existing.