New error after upgrading to 5.5

Unity Engine
1

After upgrading i was testing one of my UNET test projects, that worked previously, I suddenly receive the below error messages.

The code piece it seems to refer to is:

[Command]
    void Cmd_DoTheDeckFlip (List<GameObject> incoming_List) {

Have something changed so [command] does not support List any longer?

This works:

[Command]
    void Cmd_DoTheDeckFlip () {

I have more error messages:

2

I’ve got the same errors for UnityEngine.UI.Image and UnityEngine.Sprite. Even though both worked well before I upgraded to Unity 5.5.