NetworkServer.Spawn() only on Server (with registered prefabs) [Unity 5.2.3f1]

I've been trying to figure this out for more than a month now, I cannot proceed in my game's development due to this error which appeared all of sudden. I have no idea what caused this but now NetworkServer.Spawn() only works on Server while the Client sends an error message (not always)

Spawn scene object not found for -int-

followed by

Did not find target for sync message for -int-

Class which takes care of registering prefabs:

 public class RegisterPrefabs
 {
     public static void Register()
     {
         foreach (GameObject temp_ent in Resources.LoadAll<GameObject>(""))
         {
             if (temp_ent.GetComponent<NetworkIdentity>())
             {
                 ClientScene.RegisterPrefab(temp_ent);
                 Debug.Log(temp_ent + " registered for loading!");
             }
         }
     }
 }

Registering prefabs:

         public override void OnStartLocalPlayer()
         {
             RegisterPrefabs.Register();
         }

Quick method for testing NetworkServer.Spawn():

         public void TestSpawn(Vector2 pos, Quaternion rot)
         {
             Object[] randomWeapon = Resources.LoadAll("Guns/");
             GameObject go = Instantiate(randomWeapon[Random.Range(0, randomWeapon.Length)], pos, rot) as GameObject;
             NetworkServer.Spawn(go);
         }

Any ideas? I desperately need help. I even tried changing connection's main channel (0) to ReliableSequenced (on Host code):

             ConnectionConfig config = new ConnectionConfig();
             config.AddChannel(QosType.ReliableSequenced);
             config.AddChannel(QosType.Unreliable);
             manager.StartHost(config, 8);

But no use. Instead, NetworkServer.Destroy() seems to be working just fine on Objects the scene comes with. Every GameObject has its own NetworkIdentity (and NetworkTransport), no Server Only, no Client Auth(although I tried with, no change)