JSON Parse Err online men INTE lokalt (XAMPP)

"use strict";
// Function for faster JS code
function byId(id) {
  return document.getElementById(id);
}
// Listen for when all DOM is loaded
window.addEventListener("DOMContentLoaded", () => {
  // Grab chat window (a <div>) and clean it first
  let chatbox = byId("chatBox");
  chatbox.innerHTML = "";
  // Initiate function that updates chat window
  function writeChat(chat) {
    // Empty chat window first
    chatbox.innerHTML = "";
    // Create each chat message before output
    chat.forEach((chat) => {
      // Chat message in the form of "[DATESTAMP] username (with clickable link to profile): <message>"
      let msg = `<p>[${chat.chatmsg_created}] <a class="memberslink2" href="profile.php?user=${chat.chatmsg_by}">${chat.chatmsg_by}</a>: ${chat.chatmsg}</p>`;
      // Output to chat window
      chatbox.innerHTML += msg;
    });
  }
  // Initiate function that retrieves chat from PHP service
  function updateChat() {
    fetch("chatnotgpt.php")
      .then((response) => response.json())
      .then((data) => writeChat(data));
  }
  // Check if input box and send button exists to start listening
  // and also to not get error in console if they don't exist.
  if (byId("sheesh") && byId("chatta")) {
    let input = byId("sheesh"); // = <input type="text"> field for chat msg
    let btn = byId("chatta"); // = <input type="submit"> submit btn but it is NOT inside <form> so no actual submit effect!
    // Listen for clicks on "Posta" button
    btn.addEventListener("click", (e) => {
      // Show Alert and stop function if no message (after stripping tags)
      if (stripTags(input.value) == "") {
        alert("Skriv något först!");
        return;
      }
      // Use sendMessage function
      sendMessage(input);
    });
    // Listen for Enter key press to also work than just click
    input.addEventListener("keydown", (e) => {
      if (e.key === "Enter") {
        // Show Alert and stop function if no message (after stripping tags)
        if (stripTags(input.value) == "") {
          alert("Skriv något först!");
          return;
        }
        sendMessage(input);
      }
    });
    // Function to send message in chat window (works with BOTH Enter and mouseclick)
    function sendMessage(input) {
      // Grab current input value after stripping some tags
      // Show Alert and stop function if no message (after stripping tags)
      if (stripTags(input.value) == "") {
        alert("Skriv något GILTIGT först!");
        return;
      }
      let getMsg = stripTags(input.value);
      // Prepare form
      let msgForm = new FormData();
      msgForm.append("msg", getMsg);
      console.log(msgForm);
      // Send it!
      fetch("chatnotgpt.php", {
        method: "POST",
        body: msgForm
      })
        .then((response) => response.json())
        .then((data) => writeChat(data));
      // Clean input box
      input.value = "";

    }
    // Function that strips all tags except <strong><i><b><u>
    function stripTags(str) {
      return str.replace(/<(?!\/?(strong|i|b|u)\b)[^>]+>/g, "");
    }
  }
  // Function that updates chat every 15 seconds
  function updateChatEvery15seconds() {
    updateChat();
    setTimeout(updateChatEvery15seconds, 15000);
  }
  updateChatEvery15seconds();
});

<?php
// Include Chat + Connect classes for DB Connection
// I have no clue why I must do this and cannot just include "config.php" as usual here.
// Something about the fact it is not normal HTML being outputted?
session_start();
header("Content-Type: application/json; utf-8;");

class Chat
{
    // Private Class Variables for "Chat"
    private $mysqli; // Connected DB Connection

    // Function that stores received DB connection on its class instance
    function __construct($dbConnect)
    {
        $this->mysqli = $dbConnect;
    }

    // Function to insert new chat message
    public function insertChatMsg($msg, $user)
    {
        // Only allow 3 HTML tags in messages
        $allowed_tags = '<strong><b><i><u>';
        $msg = strip_tags($msg, $allowed_tags);
        // SQL Injection Protection
        $msg = $this->mysqli->real_escape_string($msg);
        $user = $this->mysqli->real_escape_string($user);

        // Prepare SQL Query
        $sql = "INSERT INTO chats(chatmsg,chatmsg_total,chatmsg_by) VALUES('$msg',1,'$user');";
        $result = $this->mysqli->query($sql);
    }

    // Function to retrieve current chat messages
    public function getChatMsgs(): array
    {
        // Grab all columns from chat
        $sql = "SELECT chatmsg_created,chatmsg,chatmsg_by FROM chats ORDER BY chatmsg_created DESC;";
        // Query it
        $result = $this->mysqli->query($sql);
        // And return as associative array
        return mysqli_fetch_all($result, MYSQLI_ASSOC);
    }

    // Function to update number of total chat messages
    public function updateTotalChatMsgs()
    {
        $updateRow = "UPDATE chats SET chatmsg_total=chatmsg_total+1 WHERE chatmsg_by='Nostalgitrippen';";
        $this->mysqli->query($updateRow);
    }

    // Function to update number of chat messages for current user
    public function updateTotalChatMsgsUser($username)
    {
        $updateRow = "UPDATE users SET number_of_chatposts=number_of_chatposts+1 WHERE username='$username';";
        $this->mysqli->query($updateRow);
    }
    // END OF CLASS
}
class Connect
{
    private $db;
    // Function to connect to Database by returning a mysqli-Object
    function __construct()
    {

        // Try connecting and store it in private $db;
        try {
            $this->db = new mysqli(DBHOST, DBUSER, DBPASS, DBDB);
        }
        // Catch error when trying to connect and output custom message
        catch (mysqli_sql_exception $error) {
            die("Sheesh! Database connection failed for some reason!");
        }
    }

    public function getDB()
    {
        return $this->db;
    }
}

// Function checking if user is logged in
function isLoggedIn()
{
    // Logged in since before?
    if (isset($_SESSION['username'])) {
        return true; // Return True
    } else { // Otherwise Return False
        return false;
    }
}

// Developer Mode with local DB Connection
$devmode = false;
if ($devmode) {
    // Function for reporting errors in browsers
    error_reporting(-1);
    ini_set('display_errors', 1);

    // Database Variables | DEFAULT FOR XAMPP for local Database Connections
    define("DBHOST", "localhost");   // Address to Database hostname
    define("DBUSER", "root");        // Username to log into database
    define("DBDB", "php_projekt");        // Database name to use after login
    define("DBPASS", "localhost");   // Password to log into database
}

// If not in Developer Mode, use these public DB Connection Settings
else {
    ini_set('display_errors', 1);
    ini_set('display_startup_errors', 1);
    error_reporting(E_ALL);
    define("DBHOST", "CENSUR");   // Address to Database hostname
    define("DBUSER", "CENSUR");        // Username to log into database
    define("DBDB", "CENSUR");        // Database name to use after login
    define("DBPASS", "CENSUR");   // Password to log into database
}

// Function to start Database connection which might 
// be used several times in the same PHP file
function connectDB()
{
    // Initiate Database Connection ...
    // Check Connect.class.php for more
    $connectDB = new Connect(); // Initiate DB Connection
    $db = $connectDB->getDB(); // Store that connection in a variable that is used by other classes' instances
    return $db;                // Return stored connection as its return value
}

$chatData = new Chat(ConnectDB());
// Return all current chat messages for those not logged in
$msgs = $chatData->getChatMsgs();
if (isset($_SESSION['username'])) {
    if (isset($_POST['msg'])) {
        $chatData->insertChatMsg($$_POST['msg'], $_SESSION['username']);
        $chatData->updateTotalChatMsgs();
        $chatData->updateTotalChatMsgsUser($_SESSION['username']);
        unset($_POST['msg']);
        $msgs = $chatData->getChatMsgs();
    }
    echo json_encode($msgs, JSON_PRETTY_PRINT);
} else {
    echo json_encode($msgs, JSON_PRETTY_PRINT);
}