Created a very simple true-sync scene from tutorial - still doesn't work

I created a very simple scene. It has a set of 4 controllers as UI elements.

When a button gets pushed on one computer, the controller matching that player should light up. Simple!

The code is incredibly light. I am even using Prefabs to create the player objects. However, one client says that there are 2 players connected and the other says there is only one player connected.

I have no idea why. I get this error:

KeyNotFoundException: The given key was not present in the dictionary.
System.Collections.Generic.Dictionary`2[System.Byte,TrueSync.TSPlayer].get_Item (Byte key) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:150)
TrueSync.AbstractLockstep.OnEventDataReceived (Byte eventCode, System.Object content)
TrueSync.PhotonTrueSyncCommunicator+c__AnonStorey0.<>m__0 (Byte eventCode, System.Object content, Int32 senderId) (at Assets/Standard Assets/TrueSync/Unity/PhotonTrueSyncCommunicator.cs:43)
NetworkingPeer.OnEvent (ExitGames.Client.Photon.EventData photonEvent) (at Assets/Standard Assets/Photon Unity Networking/Plugins/PhotonNetwork/NetworkingPeer.cs:2515)
ExitGames.Client.Photon.PeerBase.DeserializeMessageAndCallback (System.Byte[] inBuff)
ExitGames.Client.Photon.EnetPeer.DispatchIncomingCommands ()
ExitGames.Client.Photon.PhotonPeer.DispatchIncomingCommands ()
PhotonHandler.Update () (at Assets/Standard Assets/Photon Unity Networking/Plugins/PhotonNetwork/PhotonHandler.cs:157)


Here's *all* of the code used:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ControllerConnection : Photon.PunBehaviour {

	// Use this for initialization
	void Start () {
		PhotonNetwork.automaticallySyncScene = true;
		PhotonNetwork.ConnectUsingSettings ("v1.0");
        var player = PhotonNetwork.player;
        player.NickName = string.Format("Controller {0}", player.ID);
	}
	public override void OnJoinedLobby() 
	{
    	PhotonNetwork.JoinOrCreateRoom ("room1234", null, null);
	}
	public override void OnJoinedRoom()
	{
	}

	void OnGUI() {
		GUI.Label (new Rect(10, 10, 100, 30), "players: " + PhotonNetwork.playerList.Length);

		if (PhotonNetwork.isMasterClient && GUI.Button (new Rect (10, 40, 100, 30), "start")) {
			PhotonNetwork.LoadLevel ("NetworkedInputTest/ControlUI");
		}
	}
}

public class LightInputModule : BaseTrueSyncBehaviour {

	LightActionSet actionSet = null;

	private ServerPlayerNumber _playerNumber = ServerPlayerNumber.None;
	[Show]
	public ServerPlayerNumber PlayerNumber
	{
		get { return _playerNumber; }
		set { 
			if ( value != _playerNumber )
			{
				_playerNumber = value;
				RepickFeeback();
			}
		}
	}

	[Show]
	ControllerFeedback feedback = null; 

	void Start()
	{
		actionSet = new LightActionSet();

		_playerNumber = (ServerPlayerNumber) this.owner.Id;

		RepickFeeback();
	}

	void RepickFeeback()
	{
		if ( PlayerNumber == ServerPlayerNumber.Player1 ) 
			feedback = GameObject.Find("Player 1 Controller").GetComponent();
		else if ( PlayerNumber == ServerPlayerNumber.Player2 ) 
			feedback = GameObject.Find("Player 2 Controller").GetComponent();
		else if ( PlayerNumber == ServerPlayerNumber.Player3 ) 
			feedback = GameObject.Find("Player 3 Controller").GetComponent();
		else if ( PlayerNumber == ServerPlayerNumber.Player4 ) 
			feedback = GameObject.Find("Player 4 Controller").GetComponent();
	}

    public override void OnSyncedInput() 
    {
		TrueSyncInput.SetTSVector2(0, actionSet.LeftStickAxis.Vector);
		TrueSyncInput.SetTSVector2(1, actionSet.RightStickAxis.Vector);

		TrueSyncInput.SetBool(0, actionSet.Back.IsPressed );
		TrueSyncInput.SetBool(1, actionSet.Start.IsPressed );
		TrueSyncInput.SetBool(2, actionSet.XBox.IsPressed );

		TrueSyncInput.SetBool(3, actionSet.ButtonA.IsPressed );
		TrueSyncInput.SetBool(4, actionSet.ButtonB.IsPressed );
		TrueSyncInput.SetBool(5, actionSet.ButtonX.IsPressed );
		TrueSyncInput.SetBool(6, actionSet.ButtonY.IsPressed );

		TrueSyncInput.SetBool(7, actionSet.LeftBumper.IsPressed );
		TrueSyncInput.SetBool(8, actionSet.RightBumper.IsPressed );
		TrueSyncInput.SetBool(9, actionSet.LeftTrigger.IsPressed );
		TrueSyncInput.SetBool(10, actionSet.RightTrigger.IsPressed );

		TrueSyncInput.SetBool(11, actionSet.DPad_Up.IsPressed );
		TrueSyncInput.SetBool(12, actionSet.DPad_Left.IsPressed );
		TrueSyncInput.SetBool(13, actionSet.DPad_Down.IsPressed );
		TrueSyncInput.SetBool(14, actionSet.DPad_Right.IsPressed );

    }

    public override void OnSyncedUpdate () 
	{
        FP inputHorizontal = TrueSyncInput.GetFP (0);
        FP inputVertical = TrueSyncInput.GetFP (1);

		if ( feedback != null ) 
		{
			feedback.moveStickInput = TrueSyncInput.GetTSVector2(0).ToVector();
			feedback.cameraStickInput = TrueSyncInput.GetTSVector2(1).ToVector();

			feedback.Select = TrueSyncInput.GetBool(0);
			feedback.Start = TrueSyncInput.GetBool(1);
			feedback.Xbox = TrueSyncInput.GetBool(2);

			feedback.ButtonA = TrueSyncInput.GetBool(3);
			feedback.ButtonB = TrueSyncInput.GetBool(4);
			feedback.ButtonX = TrueSyncInput.GetBool(5);
			feedback.ButtonY = TrueSyncInput.GetBool(6);

			feedback.LeftBumper = TrueSyncInput.GetBool(7);
			feedback.RightBumper = TrueSyncInput.GetBool(8);
			feedback.LeftTrigger = TrueSyncInput.GetBool(9);
			feedback.RightTrigger = TrueSyncInput.GetBool(10);

			feedback.DPadUp = TrueSyncInput.GetBool(11);
			feedback.DPadLeft = TrueSyncInput.GetBool(12);
			feedback.DPadDown = TrueSyncInput.GetBool(13);
			feedback.DPadRight = TrueSyncInput.GetBool(14);
		}
		//Debug.LogFormat("OnSyncedUpdate ({0}, {1})", inputHorizontal, inputVertical);
    }
}

It works just fine for the controller detected - the buttons light up appropriately.... but one client says 2 people are connected and the other says 1 person is connected.

I have made this as simple as possible. What's going on?