Spawn scene object not found for 1 UnityEngine.Networking.NetworkIdentity:UNetStaticUpdate()

I have created a minimum Unity project which only contains a GameObject with my own NetworkLobbyManager Component. The code simply reads: using UnityEngine; using UnityEngine.Networking;

 public class LobbyManagerTest : NetworkLobbyManager
 {
     public void Start()
     {
         StartServer();
         StartClient();
     }
 }

I set LobbyScene to the scene containing the GameObject and PlayScene to an empty scene. I further set LobbyPlayerPrefab and GamePlayerPrefab to prefabs with empty class LobbyPlayerTest : NetworkLobbyPlayer and class PlayerTest : NetworkBehaviour respectively. All other settings are set to default.

When I now run the project in the editor, everything works fine.

Now I add a GameObject with an added Script class GameManagerTest : NetworkBehaviour.

Running the project now raises the error: "Spawn scene object not found for 1 UnityEngine.Networking.NetworkIdentity:UNetStaticUpdate()"

I searched the web for hours and wasn't able to find anything helping me here. I am new to UNET so probably I do something basic wrong, such as setting up the objects the wrong way. I tried many things such as saving the GameManager GameObject as prefab and registering it with the NetworkManager but to no avail. Adding a NetworkManager HUD at runtime, it shows:

Perhaps anyone can help me out what I'm doing wrong? Many thanks in advance!