Invalid index 433 (size is zd) + ArgumentException: Getting control x's position in a group with only x controls when doing Repaint

Hello.

I have a very strange problem and I'm not able to fix it. I have a ScriptableObject with a member of a custom class with a custom propertydrawer. I create an asset for this ScriptableObject. If I click on this asset in the editor to modify some values I get this error:

Invalid index 433 (size is zd) UnityEditor.DockArea:OnGUI()

ArgumentException: Getting control 4's position in a group with only 4 controls when doing Repaint Aborting

The error vanishes when I remove the member with the custom propertydrawer but of course it is no solution to remove it :).

So I stripped down my code to the absolute basics for debugging purpose but the errors remain and I don't have the slightest idea how to solve them.

This is my stripped down code: The ScriptableObject:

         using UnityEngine;
         using System.Collections;
         using System.Collections.Generic;
         
         public class Cart : ScriptableObject 
         {
             public LocalizedString localizedCartName; // the errors vanish if I remove this
             public Sprite     cartImage;
             public bool        isUnlocked;
         }

LocalizedString:

 using UnityEngine;
 using System.Collections;
 
 [System.Serializable]
 public class LocalizedString 
 {
     public string key;
 
     public string GetLocalizedString()
     {
         //return SmartLocalization.LanguageManager.Instance.GetTextValue(key);
         return string.Empty;
     }
 }

its propertydrawer:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using UnityEditor;
 using UnityEditorInternal;
 
 [CustomPropertyDrawer(typeof(LocalizedString))]
 public class LocalizedStringEditor : PropertyDrawer
 {
     public override float GetPropertyHeight (SerializedProperty property, GUIContent label) 
     {
         return 5.0f;
     }
 
     public override void OnGUI(Rect position, SerializedProperty property, GUIContent label) 
     {
               EditorGUI.indentLevel++;
               EditorGUILayout.LabelField("en", "bla");
               EditorGUI.indentLevel--;
     }
  }

This is it. Any idea what is going wrong here?

Edit: I figured out that the problem is using EditorGUILayout. Everytime I use it I get these errors. When I switch to EditorGUI it works like a charm. But why is this. I'd like to use EditorGUILayout as it is far easier to use,