Getting 502 error in wordpress

In my wordpress website I have to call a PHP function after some interval from a javascript function.
Some times it works fine. But some time throws 502 error code.
Can you please explain why this type of error throws ? And how can I fix this error ?
My javascript function :

function set_online_user(){

clearInterval(setOnlineUser);
if(setUserOnline == true)
{
    setUserOnline = false;
    jQuery.ajax({
            type: 'POST',
            url: WQChat.ajaxurl,
            data: ({action : 'my_action_set_user_online'}),
            success: function(response) {
                    print_in_console("set_online_user"+response);
                    setUserOnline = true;
                    setOnlineUser = setInterval(function(){ set_online_user(); }, 1000);
                },
            statusCode: {
                502: function (error) {
                    console.log(error);
                }
            }
        });
}
}
var setOnlineUser = setInterval(function(){ set_online_user(); }, 1000);

And my PHP function is :

function my_action_set_user_online()
{
global $wpdb;
$currentUserId = get_current_user_id();
$time = current_time( $type = 'timestamp', $gmt = 1 );
if($currentUserId)
{
    update_user_meta( $currentUserId, 'last_login_on', $time);  
}
if (time_nanosleep(0, 50000000) === true) {
    echo $currentUserId;
}
die();
}

Thanks in advance..