RectTransform.position of Panel always 0 in FullHD

Hey folks,

I have a problem I can't figure out how to solve. Maybe I'm using a wrong approach here. But I want to position a menu a bit below the mouse cursor. For that I'm using RectTransform which works fine in most cases.
The case where it is not working fine is when I'm using the reference resolution. My reference resolution is 1920x1080. If I'm playing my game in that resolution, no matter if build or within Unity, the menu appears in the middle of the screen. After I set the position of my Panel it is always 0. If I use a different resolution or change my reference resolution the menu appears where I expect it.

 RectTransform myRECTTransform = myGameMenu.transform.Find("Menu").GetComponent<RectTransform>();
 myRECTTransform.position = new Vector2(Input.mousePosition.x, Input.mousePosition.y - 125);

I'm using Unity 5.1.1f1 (64bit).
Is there something I'm doing wrong or is there a bug in Unity? Google couldn't help me further. Thanks for any hints!