UNET OnTriggerEnter called due to placement of Network-GameObjects on Clients

In the multiplayer scene I have a platform, consisting of these components: NetworkTransform, NetworkIdentity (bool Local) and a PlatformController. It can be moved when a player is entering a triggerCollider-GameObject.

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The triggerCollider-GameObject has an Activator script with an UnityEvent, that is fired upon OnTriggerEnter(). I drag the corresponding Platform on its UnityEvent-Slot and selecting its MoveFunction which moves the platform and increments the waypointIndex by 1.

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This works good when both players connect simultanuesly. However,

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if the host is on the trigger that had activate the platform to move at waypoint 1 and client joins afterwards, the platform of the Client application jitters immediatly. This is caused due to the Host-GameObject instantiation which is positioned at the triggerCollider thus calling OnTriggerEnter() on its Activator script again which moves the platform.

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How do I block OnTriggerEnter when joining? Turning off the trigger-collider on the client side is not a good solution because the client could never activate the platform.

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I send the waypointindex of the platform as an SyncVar calling Cmd -> Rpc to syncronize the targetwaypoint of the platform for all players but that doesnt help removing the jittering.

 public class PlatformNetwork : NetworkBehaviour { 

     [SyncVar]
     public int waypointIndex;
     private Platform p;

     private void Awake(){
         p = GetComponent<Platform> ();
     }
     //TODO Causes jitter of Platform at Client if Host on PlatformTrigger, because Host-GO is created in same position of PlatFormTrigger which calls OnTriggerEnter() -> leading to misbehaviour at Client
     private void Update () {
         if (!isLocalPlayer)
             return;

         Cmd_Data(waypointIndex = p.waypointIndex);
     }

     [Command] //Called from client, run on Server
     public void Cmd_Data(int myIndex) {
         Rpc_Data(myIndex);
     }

     [ClientRpc]//ClientRpc calls - which are called on the server and run on clients.
     private void Rpc_Data(int myIndex) {
         p.waypointIndex = myIndex;
     }
 }