GoogleVR Sprite2D and Buttons Flickering/Glitch and FPS Canvas on one Screen

Good evening people,

I am building a vr application for my thesis. I almost finished it, installed it on 3 devices and I encountered the following problem. The buttons on the main menu and the "wall paint" are glitching and flickering. The buttons are children of a Panel which is a children of a Canvas and they have as a child a TextMesh text. The wall paint is a prefab of a paint which has a Canvas as a child and this Canvas has an UI -> Image child.

Also the fps counter is showing only on one screen (althought this is not a big problem as it will not be in the final app).

I am running Unity 5.5.0f3 and GoogleVR SDK 1.0.3. Any help would be greatly appreciated.