mainmenuscript keeps crashing the game

So i have made this mainmenu-script which works perfectly fine, if u start in mainmenu and then load a lv. Going from any lv to mainmenu works fine too. But if go from a lv to mainmenu and then try to access any lv the game crashes. I tried modifying it a bit, but that made it worse, so i reset to a way i know it works at least a bit. Can someone tell what's causing the bug?

Maybe some additional bug behaivour note, once the bug is caused and everthing frozen, u can simply, pause and unpause to get a gameover screen, which reloads same lv and suddenly its fine.

Here the code i wrote:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.SceneManagement;
 using UnityEngine.UI;
 
 
 public class MainmenuManager : MonoBehaviour {
 
     //CanvasGroup HUDCanvasgroup;
 
     // Use this for initialization
     void Start () {
 
         //GameObject HUDCanvas = GameObject.Find ("HUDCanvas");
         //HUDCanvasgroup = HUDCanvas.GetComponent<CanvasGroup> ();
     }
     
     // Update is called once per frame
     void Update () {
     
         if (Input.GetKeyUp (KeyCode.Escape)) {
             quit();
 
         }
     }
     
     // loads the lv with number lv
     public void loadLevel(int lv){
         SceneManager.LoadScene (lv);
 
     }
         
     //ends the programm
     public void quit(){
         Application.Quit();
     }
 
     public void gotoMainmenu(){
         SceneManager.LoadScene ("Mainmenu");
             }
 
 }