Loading scenes from my menu breaks a script that usually works...

Okay, first of all sorry for the title, I just really don't know how to describe the problem in a chort manner.

The t$$anonymous$$ng is, I have multiple levels in a tower-defense-like game and inside of each level I have GameObject WorldSpawner with a script enemySpawn, responsible for, you guessed it, spawning the enemies.

The WorldSpawner also has a script called SceneChanger that changes my current scene by pressing the page up or page down key, loading the next or previous scene respectiveley. T$$anonymous$$s was for testing purposes, works perfectly fine and I am currently working on the UI, including a main- and levelselection-menu.

But for whatever reason, when I load a level via the scripts for my menu buttons the enemies won't spawn and I can't seem to figure out what the big difference between the two scene-loading scripts is that breaks it.

I'll post the scripts down here and would really appreciate any help you guys n' girls can offer. :)

T$$anonymous$$s is the SceneChanger-Script:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.SceneManagement;
 
 public class SceneChanger : MonoBehaviour
 {
     private int currentScene;
     // Use t$$anonymous$$s for initialization
     void Start()
     {
         currentScene = SceneManager.GetActiveScene().buildIndex;
     }
 
     // Update is called once per frame
     void Update()
     {
         if (Input.GetKey("page up"))
         {
             SceneManager.LoadScene(currentScene + 1);
         }
         else if(Input.GetKey("page down"))
         {
             SceneManager.LoadScene(currentScene - 1);
         }
     }
 }

T$$anonymous$$s is the LevelselectionMenu-Script:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.SceneManagement;
 using UnityEngine.UI;
 public class LevelselectionMenu : MonoBehaviour
 {
     public Button level1But;
     public Button level2But;
     public Button level3But;
     public Button level4But;
     public Button level5But;
     public Button backBut;
 
     void Start()
     {
         level1But = level1But.GetComponent<Button>();
         level2But = level2But.GetComponent<Button>();
         level3But = level3But.GetComponent<Button>();
         level4But = level4But.GetComponent<Button>();
         level5But = level5But.GetComponent<Button>();
         backBut = backBut.GetComponent<Button>();
     }
 
     public void level1Press()
     {
         SceneManager.LoadScene("Scenes/Scene1");
     }
     public void level2Press()
     {
         SceneManager.LoadScene("Scenes/Scene2");
     }
     public void level3Press()
     {
         SceneManager.LoadScene("Scenes/Scene3");
     }
     public void level4Press()
     {
         SceneManager.LoadScene("Scenes/Scene4");
     }
     public void level5Press()
     {
         SceneManager.LoadScene("Scenes/Scene5");
     }
     public void backButPress()
     {
         print("backButPressed");
         SceneManager.LoadScene("Scenes/MainMenu");
     }
 }

And finally, here's the EnemySpawn-Script(what's needed for the first level anyways, sorry t$$anonymous$$s whole t$$anonymous$$ng has become so long but I wanted to give you lots of information):

 using UnityEngine;
 using UnityEngine.SceneManagement;
 using System.Collections;
 using System;
 
 public class EnemySpawn : MonoBehaviour
 {
     private GameObject enemy;
     private int enemyType;
     private int laneID;
     private int goodOrBad;
     private int enemiesSpawned;
     private int waveSize;
     private int getZ;
     private float zKoordinate;
     private float lane;
     private string currentScene;
     private Vector3 enemySpawnPoint;
     private DateTime firstStamp;
     private DateTime secondStamp;
     private bool timePassed;
 
     // Use t$$anonymous$$s for initialization
     void Start()
     {
         currentScene = SceneManager.GetActiveScene().name;
         Debug.Log(currentScene);
         switch (currentScene)
         {
             case "Scene1":
                 waveSize = 20;
                 spawnWave1();
                 break;
             case "Scene2":
                 waveSize = 30;
                 spawnWave2();
                 break;
             case "Scene3":
                 waveSize = 30;
                 spawnWave3();
                 break;
             case "Scene4":
                 waveSize = 35;
                 spawnWave4();
                 break;
             case "Scene5":
                 waveSize = 50;
                 spawnWave5();
                 break;
         }
 
         enemiesSpawned = 0;
         firstStamp = DateTime.Now;
     }
     // Update is called once per frame
     void Update()
     {
         secondStamp = DateTime.Now;
         if (secondStamp >= firstStamp.AddSeconds(1.0) && enemiesSpawned < waveSize)
         {
             switch (currentScene)
             {
                 case "Scene1":
                     spawnWave1();
                     break;
                 case "Scene2":
                     spawnWave2();
                     break;
                 case "Scene3":
                     spawnWave3();
                     break;
                 case "Scene4":
                     spawnWave4();
                     break;
                 case "Scene5":
                     spawnWave5();
                     break;
             }
             firstStamp = DateTime.Now;
         }
         else if (enemiesSpawned > waveSize && GameObject.FindGameObjectWithTag("Enemy") == null)
         {
             Debug.Log("Sie haben gewonnen!");
             //AppHelper.Quit();
         }
     }
     public void spawnWave1()
     {
         laneID = (int)UnityEngine.Random.Range(1, 5);
         switch (laneID)
         {
             case 1:
                 lane = -15.0f;
                 break;
             case 2:
                 lane = -5.0f;
                 break;
             case 3:
                 lane = 5.0f;
                 break;
             case 4:
                 lane = 15.0f;
                 break;
         }
         enemy = Resources.Load("Enemies/Asteroid", typeof(GameObject)) as GameObject;
         enemy = Instantiate(enemy);
         enemySpawnPoint = new Vector3(lane, 11.0f, 70.0f);
         enemy.transform.position = enemySpawnPoint;
         enemiesSpawned++;
     }
 }