Null Reference Exception when finding GameObject on Networked Game

I'm making a first person shooter using Unity Networking, which is something I've never used before, so I'm still trying to make sense of it. I have a GameObject in my Network Scene which is called DecalManager, and it is being used for the creation of things like bullet hole decals. There is a script attached to it which is a NetworkBehaviour, and contains data such as a list of all current bullet holes, to manage them and make sure there are no more than a certain amount on the server.

My issue is this; when I was using a Network Manager, it was working fine. The local players all store a reference to the servers decal manager, and when a gunshot hits a certain type of object, it calls a function in the decal manager to create a new one and the correct position. But then, I swapped it out for a Network Lobby Manager, and suddenly I'm getting a Null Reference Exception when it tries to find the reference. After some experimentation, I've determined that it is unable to find the DecalManager GameObject, even though I can definitely see that it is in the scene.

I have tried swapping the Network Lobby Manager back to a Network Manager to make sure I haven't changed anything else that would have broken it, and it started working again. So somehow, the simple act of using a Network Lobby Manager instead of a standard Network Manager has made it unable to find the object in the scene.

The relevant code in the player script, which is a Network Behaviour attached to all players:

 private DecalManager decalManager;
 
 void Start(){
             //This is the line that gives the null reference exception
         decalManager = GameObject.FindGameObjectWithTag("DecalManager").GetComponent<DecalManager> ();
 }

The decal manager script:

 using UnityEngine;
 using UnityEngine.Networking;
 using System.Collections;
 using System.Collections.Generic;
 
 public class DecalManager : NetworkBehaviour {
 
     [SerializeField] private int maxBulletHoles = 100; //Max amount of decals that can exist
     [SerializeField] private GameObject bhSprite; //Sprite image for bullet holes
 
     private List<GameObject> bulletHoles = new List<GameObject>();
 
     public void CreateBullethole(Vector3 position, Quaternion rotation)
     {
         if(bulletHoles.Count >= maxBulletHoles) //Has the maximum been reached?
         {
             NetworkServer.Destroy (bulletHoles[0]); //Destroy decal on server
             Destroy (bulletHoles[0]); //Destroy object locally
             bulletHoles.RemoveAt (0); //Remove from list
         }
 
         GameObject newHole = (GameObject)Instantiate (bhSprite, position, rotation); //Create new decal
 
         float randRot = Random.Range (0, 360);
         float randScale = Random.Range (0, 1);
 
         newHole.transform.RotateAround(newHole.transform.position, newHole.transform.forward, randRot);
 
         //TODO: MAKE DECALS APPEAR WITH SIZE VARIATIONS
 
         newHole.transform.parent = transform; //Set parent
         NetworkServer.Spawn(newHole); //Spawn on server
         bulletHoles.Add (newHole); //Add to array
     }
 }

Does anybody know why this could be happening?

Thanks for any assistance in advance.

Update: I have also found that this error is not occurring for clients connecting to the game, only for the player that is hosting the server.