Unity Unet spawnig Fail,Unity Unet NetworkManager spawning fail

So I am working on a simple Multiplayer-FPS, in which I am spawning my Players with the "Auto create Player" option of the NetworkManager.
My first Problem is:
It gives me the Error Statement: GameObject Player is a prefab, it can't be spawned. This will cause errors in builds. UnityEngine.Networking.NetworkManagerHUD:OnGUI()

And my second Problem is:
NullReferenceException: Object reference not set to an instance of an object UnityEngine.Networking.NetworkIdentity.OnStartAuthority () (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkIdentity.cs:385) UnityEngine.Networking.NetworkIdentity.ForceAuthority (Boolean authority) (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkIdentity.cs:119) UnityEngine.Networking.NetworkServer.SpawnObjects () (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkServer.cs:1793) UnityEngine.Networking.NetworkManager.StartServer (UnityEngine.Networking.Match.MatchInfo info, UnityEngine.Networking.ConnectionConfig config, Int32 maxConnections) (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkManager.cs:368) UnityEngine.Networking.NetworkManager.StartServer (UnityEngine.Networking.Match.MatchInfo info) (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkManager.cs:290) UnityEngine.Networking.NetworkManager.StartServer () (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkManager.cs:285) UnityEngine.Networking.NetworkManager.StartHost () (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkManager.cs:549) UnityEngine.Networking.NetworkManagerHUD.OnGUI () (at C:/buildslave/unity/build/Extensions/Networking/Runtime/NetworkManagerHUD.cs:88)

The fist thing causes my Player not to spawn in Builds, as the Host and the Client and the second causes it to have the same problem inside of the Unity Game window.
Please help me with this! I did not found any tutorials on this.
Thank You for your help

PS: These problems came with the new Unity 2017.1 Update.

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