AJAX http request not returning when readyState is 4 = DONE, returns at 3

I have javascript client code making this http request call (including only the relevant code section):

...
var data = new FormData();
data.append('prog', x);
data.append('userid', <?php echo $id; ?>);
data.append('usecache', usecache);

var xmlhttp = new XMLHttpRequest();
xmlhttp.open('POST', 'solveIt2.php', true);
xmlhttp.onreadystatechange = function() {
    document.getElementById("solve_button").textContent = "Solve";
    if (this.readyState == 4 && this.status == 200) {
        document.getElementById("yy").innerHTML = this.responseText;                     
    } else {
        document.getElementById("yy").innerHTML = "ERR1 (" + this.readyState + ", " + this.status + ") <> (4, 200)";
    }
};
xmlhttp.send(data);   

The server side solveIt2.php code is simply:

<?php
echo "blabla";
return;
?>

When looking at my html page I'm getting this: ERR1 (3, 200) <> (4, 200)

According to Javascript documentation XMLHttpRequest.readyState 3 means ==> LOADING Downloading; responseText holds partial data.

Why don't I get the http repose generated by the server? Do I need to write my client code differently and wait for state done only? how?

source https://stackoverflow.com/questions/73939769/ajax-http-request-not-returning-when-readystate-is-4-done-returns-at-3